Free-space optical transmission can provide high-speed links for a variety of applications. However, inhomogeneities in the temperature and pressure of the atmosphere lead to variations of the refractive index along the transmission path. This atmospheric turbulence can deteriorate the quality of the image formed at the receiver, and can cause fluctuations in both the intensity and the phase of the received signal. Laser beams experience three effects under turbulence: beam wander, beam spreading and scintillation. Of the three turbulence effects, free space optical systems might be most affected by scintillation. Random interference with the wave front can cause peaks and dips, resulting in receiver saturation or signal loss. Based on Kolmogorov’s theory, the Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER) from scintillation were drawn in this paper. For a FSO system, after choosing a certain beam and communication range, link performance characterized by its SNR or BER is related to the refractive index structure coefficient Cn2 and turbulence depth Lt. Considering all the three effects, we find for a weak turbulence model, a FSO system should be applied within a communication range of 2.5km and above the ground at least 5m.