Hydrogenated amorphous carbon (a-C:H) thin films have been prepared by rf glow discharge decomposition of C6H5CH3 gas at 13.6 MHz. Multi-energy boron or phosphorus implantation into these films has been performed to achieve a relatively flat impurity profile and the doping effect as a function of implanted impurity concentration has been studied. It is found that there are critical values of the implanted boron and phosphorus concentration below which there is no significant change observed in the electrical conductivity activation energy Ea, and the room temperature conductivity σRT increases only slightly with the impurity concentration. When the critics values are reached, there are first an abrupt decrease in Ea and an abrupt increase in σRT. Then Ea decreases and σRT increases gradually with further increase in impurity concentraton. The values of the critical concentrations are also found to be dependent on the substrate temperature during deposition. The critical concentrations are lower for lower deposition substrate temperatures. Preliminary results on the comparison between the doping effects by B+ and BF2 implantation show that the doping efficienty by BF2 implantation is much higher.