A communication device for handling data transmissions after detecting a failure, comprising: a storage device, storing instructions of: receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first BS; transmitting a first plurality of protocol data units, PDUs, of the first flow over the first DRB to the first BS in response to the first RRC message; receiving a PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a second plurality of PDUs of the first flow over the second DRB to the first BS in response to the PDU; detecting a failure, wherein the failure is a radio link failure, a handover failure, an integrity check failure or a RRC connection reconfiguration failure; transmitting a failure recovery request message to the first BS or a second BS in response to the failure; receiving a failure recovery message from the first BS or the second BS in response to the failure recovery request message; and transmitting a third plurality of PDUs of the first flow over the first DRB to the first BS or the second BS in response to the failure recovery message, after recovering the failure; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device; wherein the storage device further stores the instructions of: releasing a secondary cell, SCell, configuration or a secondary cell group, SCG, configuration in response to the failure; configuring the communication device with an identifier for monitoring a control channel transmitted by the first BS for scheduling at least one of an uplink, UL, transmission and a downlink, DL, transmission, before detecting the failure; and keeping the identifier in response to the failure. a storage device, storing instructions of: receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first BS; transmitting a first plurality of protocol data units, PDUs, of the first flow over the first DRB to the first BS in response to the first RRC message; receiving a PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a second plurality of PDUs of the first flow over the second DRB to the first BS in response to the PDU; detecting a failure, wherein the failure is a radio link failure, a handover failure, an integrity check failure or a RRC connection reconfiguration failure; transmitting a failure recovery request message to the first BS or a second BS in response to the failure; receiving a failure recovery message from the first BS or the second BS in response to the failure recovery request message; and transmitting a third plurality of PDUs of the first flow over the first DRB to the first BS or the second BS in response to the failure recovery message, after recovering the failure; and receiving a first radio resource control, RRC, message configuring a first data radio bearer, DRB, and configuring a mapping that a first flow is mapped to the first DRB, from a first BS; transmitting a first plurality of protocol data units, PDUs, of the first flow over the first DRB to the first BS in response to the first RRC message; receiving a PDU of the first flow or a second flow over a second DRB from the first BS; transmitting a second plurality of PDUs of the first flow over the second DRB to the first BS in response to the PDU; detecting a failure, wherein the failure is a radio link failure, a handover failure, an integrity check failure or a RRC connection reconfiguration failure; transmitting a failure recovery request message to the first BS or a second BS in response to the failure; receiving a failure recovery message from the first BS or the second BS in response to the failure recovery request message; and transmitting a third plurality of PDUs of the first flow over the first DRB to the first BS or the second BS in response to the failure recovery message, after recovering the failure; and a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device; wherein the storage device further stores the instructions of: releasing a secondary cell, SCell, configuration or a secondary cell group, SCG, configuration in response to the failure; configuring the communication device with an identifier for monitoring a control channel transmitted by the first BS for scheduling at least one of an uplink, UL, transmission and a downlink, DL, transmission, before detecting the failure; and keeping the identifier in response to the failure. releasing a secondary cell, SCell, configuration or a secondary cell group, SCG, configuration in response to the failure; configuring the communication device with an identifier for monitoring a control channel transmitted by the first BS for scheduling at least one of an uplink, UL, transmission and a downlink, DL, transmission, before detecting the failure; and keeping the identifier in response to the failure.