An arrangement in a rock drilling machine, which arrangement comprises: a rock drilling machine comprising a gear module, which is located at a front end portion of the rock drilling machine and a percussion module, which is located at a rear end portion of the rock drilling machine, and the gear module and the percussion module are on a same axial line and wherein a body of the gear module comprises a first axial joint surface facing towards the rear end portion of the rock drilling machine, and whereby a body of the percussion module comprises a second axial joint surface facing towards the front end portion of the rock drilling machine; connecting means (27, 30, 31, 34) for connecting the gear module and the percussion module; and a carriage supportable movably by a feed beam and comprising a body part, which is an elongated element arranged longitudinally relative to the rock drilling machine and axial first support surfaces for connecting the rock drilling machine; characterized in that the carriage further comprises two transverse fastenings lugs which are located at a transverse distance from each other and are provided with the axial first support surfaces facing towards the front end portion of the rock drilling machine; the percussion module is located between the fastening lugs at the axial line of the carriage; and the connecting means comprise a plurality of first connecting means (27, 31) and a plurality of second connecting means (30, 34), wherein the first connecting means (27, 31) are capable of assembling the rock drilling machine, by connecting the gear module and the percussion module axially successively to each other, by generating a first connecting force for pressing the first axial joint surface of the gear module and the second axial joint surface of the percussion module towards each other, and the second connecting means (30, 34) are capable of mounting the pre-assembled rock drilling machine to the carriage, by generating a second connecting force for pressing the first axial joint surface of the gear module and the second axial joint surface of the percussion module towards each other, and which second connecting force is configured to fasten the gear module and the percussion module to the axial first support surfaces of the carriage. a rock drilling machine comprising a gear module, which is located at a front end portion of the rock drilling machine and a percussion module, which is located at a rear end portion of the rock drilling machine, and the gear module and the percussion module are on a same axial line and wherein a body of the gear module comprises a first axial joint surface facing towards the rear end portion of the rock drilling machine, and whereby a body of the percussion module comprises a second axial joint surface facing towards the front end portion of the rock drilling machine; connecting means (27, 30, 31, 34) for connecting the gear module and the percussion module; and a carriage supportable movably by a feed beam and comprising a body part, which is an elongated element arranged longitudinally relative to the rock drilling machine and axial first support surfaces for connecting the rock drilling machine; characterized in that the carriage further comprises two transverse fastenings lugs which are located at a transverse distance from each other and are provided with the axial first support surfaces facing towards the front end portion of the rock drilling machine; the percussion module is located between the fastening lugs at the axial line of the carriage; and the connecting means comprise a plurality of first connecting means (27, 31) and a plurality of second connecting means (30, 34), wherein the first connecting means (27, 31) are capable of assembling the rock drilling machine, by connecting the gear module and the percussion module axially successively to each other, by generating a first connecting force for pressing the first axial joint surface of the gear module and the second axial joint surface of the percussion module towards each other, and the second connecting means (30, 34) are capable of mounting the pre-assembled rock drilling machine to the carriage, by generating a second connecting force for pressing the first axial joint surface of the gear module and the second axial joint surface of the percussion module towards each other, and which second connecting force is configured to fasten the gear module and the percussion module to the axial first support surfaces of the carriage.