In this research project we are concentrating on investigating the losses affecting a Free Space Optical communication system, which is considered as a very secure communication system. Losses are mainly atmospheric attenuation and geometric losses. The several types of atmospheric attenuation: absorption, scattering and scintillations are introduced. Several types of absorption molecules which causes the most absorption at specific wavelengths are given. Also we have investigated the possibility of neglecting the attenuation due to absorption by choosing the suitable wavelength. Attenuation due to scattering are introduced and treated. Relationships which describe Rayleigh scattering and Mie scattering are also given. The effect of each type of scattering on an FSO are discussed and why we can neglect Rayleigh scattering is justified. Analytical study which was carried out according to experimental data was taken from the Civil Aviation AND Meteorology Authority - Yemen Meteorological Service. The Mie scattering coefficient or aerosols scattering are calculated at several ranges of visibilities. The scattering coefficient due to rain is calculated as a Non-selective scattering. The attenuation due to hazy days and rainy days are also calculated. The analytical study also discuss the variation of geometric loss according to the variation of transmitter, receiver aperture and the beam divergence. The total attenuation is calculated. Finally, a miniature optical system with a very high degree of data security was build and successfully tested.