Effect of running-in process on friction behaviour of carbon nitride (CNx) coating in N2 gas stream was investigated with a newly introduced two-step ball-on-disk friction test, where the rubbed Si3N4 ball in the pre-sliding (step 1) was replaced by a new CNx-coated Si3N4 ball in the subsequent sliding stage under N2 gas (step 2). The two-step friction test is clarified to be a simple but effective technique for obtaining contact material combination of self-mated CNx coatings and for achieving stable and low frictions of CNx coatings. Friction coefficients of CNx/CNx in N2 gas stream decrease greatly from 0.07 without pre-sliding to less than 0.025 in two-step friction tests. The minimum friction coefficient of 0.004 was obtained by introducing 500 cycles of pre-sliding in ambient air. These stable and low frictions are attributed to the generation of self-mated CNx coatings and the formation of a lubricious layer on the disk surface.