The multiple beam interferometer first constructed by the French scientists Charles Fabry and Alfred Perot in the late eighteen hundreds (1899), is of considerable importance in modern optics. It is particular value arises from the fact that besides being a spectroscopic device of extremely high resolving power; it also serves as the basic laser resonant cavity. A study of the change of transmission with respect to the change of phase difference and study the effect of change of reflectivity, type of material such as(GaAs, MgF2, quartz, Silica, Ge and NaCl)and gap width on the value of transmission. The results are computed using computer program that is written by Matlab 6.5. The transmission value depends on reflectivity and show the transmission increases with the decreasing of reflectivity, the maximum value of transmission reach 100% approximately for all reflectivity. show also the value of finesse depends on reflectivity and the relation between finesse reflectivity is proportion when the reflectivity increases the finesse increase also the transmission value depends on wavelength depending on the refractive index and show how the transmission varies with wavelength for different types of materials, also show how the transmission value depends on gap width and the results studied for different types of materials.