Important role of transfer layer from CNx coatings to Si_3N_4 ball were already proposed in the previous our study on ultra low friction of CNx coating sliding against Si_3N_4 ball in Nitrogen. In the present study, in order to know the effect of the presence of CNx coating to the mating surface, CNx coated Si_3N_4 balls with various thickness of CNx were prepared and used for pin-on-disk friction test against CNx coatings in Nitrogen. As a result, minimum friction coefficient of 0.013 was obtained in the case of 20nm of thickness of CNx on the ball. It was confirmed that transferred CNx on the Si_3N_4 ball is more effective than the coated CNx for ultra low friction in Nitrogen.