We studied the second and thitd order dispersion of the dielectric cavity mirror in femtosecond Pulsed lasers. The amount of dispersion can be a function of its construction Parameters and the angle of incidence. The second and third order dispersion iS increased with the ratio nH/nL and the number of lapers. The thied order dispersion begins to oscillate when the ratio nH/nL and the number of layers are in excess of 1.67 and 25-layer respectively. Moreover, we conclude that the effect of incident angle on the third order dispersion is very important.