Turbulence effects on the performance of M'ary PPM format optical communication channel using an APD-based receiver are investigated. The bit error probability vs. received signal has been computed for no turbulence, mild turbulence ( σ =0.2), and strong turbulence (σ =0.4). Calculations were done for a typical APD characteristics, background power of 0.25 nW, integrating time of 2 nsec, and signal power in the range of 5-50 nW. throughout the calculation, Gaussian statistics were assumed. A set of curves, describing the bit error probability vs. received power were generated. From these curves, the turbulence induced loss factors were estimated. The values of these factors are approximately 1.3 for mild and 2-2.5 for strong turbulence. for a given signal power, the increase in the bit error probability due to turbulence may amount to three orders of magnitude.