The invention relates to an arrangement in a rock drilling machine and to a method of mounting the same. The rock drilling machine (4) comprises a gear module (13) and a percussion module (15), which are arranged on a same axial line (17). The modules are pressed towards each other by means of several first connecting means (27) and several second connecting means (30). The connecting means generate first axial connecting forces (F1) and second axial connecting forces (F2) which receive axial operational forces of the rock drilling machine. The second connecting means also fasten the rock drilling machine to a carriage (5) provided with support surfaces (28).