Abstract Electromagnetic shielding of nylon-66 composites applied to laser modules was studied experimentally and theoretically. The effects of conductive carbon fiber length and weight percentage upon the shielding effectiveness (SE) of nylon composites were investigated. The result showed that the SE of long carbon fiber filled nylon-66 composites was found to be higher SE than short carbon fiber filled nylon-66 composites under the same weight percentage of carbon fibers. In addition, higher electromagnetic shielding was obtained for the composite with higher contents of carbon fibers at the same length. The SE of conductive carbon fiber filled nylon-66 composites was measured to be 41 dB at low frequency of 30 MHz and 59 dB at high frequency of 1.5 GHz. The results of SE predicted by the proposed theoretical model and the results measured by experiments were in good agreement with each other for carbon fibers filled nylon-66 composites of different lengths. The effects of fiber orientation on SE of nylon and LCP composites were also investigated. The result showed that the SE of LCP composites was found to be higher than nylon composites under the same weight percentage of carbon fiber. This is due to that the fiber orientation in LCP composites attempts to keep the same direction.