A stroller frame and a foldable and detachable chair frame for mounting on the stroller frame, including: a pair of sockets connected to the stroller frame spaced apart from each other; a chair frame having a pair of connectors mounted on both sides thereof for detachably coupling the chair frame to the stroller frame by inserting the connectors into the pair of sockets respectively; wherein the chair frame further comprising: a backrest section, a seat section, and a pair of lockable joints lockably and pivotably connecting the backrest section and the seat section; wherein the chair frame has a deployed configuration by locking the lockable joints to retain the backrest section in a first position, and a folded configuration by unlocking the lockable joints to permit the backrest section to rotate to a second position; and a release actuator for unlocking the pair of lockable joints thereby permitting backrest section to rotate from the first position to the second position, wherein the connectors each being formed with a plurality of first position indents, and the lockable joints each including: a plate pivoted to the connectors by a pivot element, the plate being formed with a plurality of second position indents corresponding to the first position indents; an upper joint part, having a portion connected with the backrest section, and a portion pivoted between the connectors and plate and formed with third position indents on one side thereof; a lower joint part, having a portion connected with the seat section, and a portion pivoted between the connectors and plate and formed with fourth position indents on one side thereof; and a positioning mechanism, operatively mounted on the chair frame and releasably engaging with the third position indents, fourth position indents, and one of the first position indents and the second position indents thereby positioning the chair frame in a selected inclination position. a pair of sockets connected to the stroller frame spaced apart from each other; a chair frame having a pair of connectors mounted on both sides thereof for detachably coupling the chair frame to the stroller frame by inserting the connectors into the pair of sockets respectively; wherein the chair frame further comprising: a backrest section, a seat section, and a pair of lockable joints lockably and pivotably connecting the backrest section and the seat section; a backrest section, a seat section, and a pair of lockable joints lockably and pivotably connecting the backrest section and the seat section; wherein the chair frame has a deployed configuration by locking the lockable joints to retain the backrest section in a first position, and a folded configuration by unlocking the lockable joints to permit the backrest section to rotate to a second position; and a release actuator for unlocking the pair of lockable joints thereby permitting backrest section to rotate from the first position to the second position, a release actuator for unlocking the pair of lockable joints thereby permitting backrest section to rotate from the first position to the second position, wherein the connectors each being formed with a plurality of first position indents, and the lockable joints each including: a plate pivoted to the connectors by a pivot element, the plate being formed with a plurality of second position indents corresponding to the first position indents; an upper joint part, having a portion connected with the backrest section, and a portion pivoted between the connectors and plate and formed with third position indents on one side thereof; a lower joint part, having a portion connected with the seat section, and a portion pivoted between the connectors and plate and formed with fourth position indents on one side thereof; and a positioning mechanism, operatively mounted on the chair frame and releasably engaging with the third position indents, fourth position indents, and one of the first position indents and the second position indents thereby positioning the chair frame in a selected inclination position. a plate pivoted to the connectors by a pivot element, the plate being formed with a plurality of second position indents corresponding to the first position indents; an upper joint part, having a portion connected with the backrest section, and a portion pivoted between the connectors and plate and formed with third position indents on one side thereof; a lower joint part, having a portion connected with the seat section, and a portion pivoted between the connectors and plate and formed with fourth position indents on one side thereof; and a positioning mechanism, operatively mounted on the chair frame and releasably engaging with the third position indents, fourth position indents, and one of the first position indents and the second position indents thereby positioning the chair frame in a selected inclination position.