In this thesis we have investigated the effect of Mo +6 subtitution and preparation conditions on the structural, magnetic and electrical properties and phase separation phenomena of La 0.5 Ca 0.5 Mn 1-x Mo x O 3 (0? x ?0.08) and Pr 0 . 5 Sr 0.5 MnO 3 manganites. The samples of La 0.5 Ca 0.5 Mn 1-x Mo x O 3 have been prepared by the standars solid stste reaction method for investigation of phase separation and structural, magnetic and electrical properties. After of preparing pellet the samples divided into two series by cooling rate and without cooling rate and sintered. The analysis of the XRD pattern of samples that hade cooling rate and samples that hade note cooling rate(x=0.04, 0.06, 0.08)showed impurity peaks with Reitveld analysis that note all of compounds hade orthorhombic structure with space group m. By increasing of doping lattice parameters and unit cell voloum in effect of enterance of Mn +2 hade not regular behavior For samples of by cooling rate and without cooling rate.Thus this increasing of lattice parameters and unit cell voloum were the reason of entrance of Mn +2 with ionic radii of Mn +2 Mn +3 when that Mo substituted for Mn and above it, we have a impurity phase, CaMoO 4 . So this increasing of lattic parameters and unit cell volume in x=0.06,0.08 were the reason of increasing of deficiency in Ca site. The SEM images for samples of that by cooling rate was given. Ac magnetic susceptibility of samples observed two transitions that ones in (T co ) between temperatures 240-270K and ones in between temperatures (T c )140-170K, that with increasing of doping in samples of by cooling rate to x=0.04, decreased curie temperature and then increased and in samples of without cooling rate to x=0.02 decreased T c and then increased. In sample of x=0.02 that there was samples coexistence of ferromagnetic and charge orderingantiferromagnetic phases. In sample of with cooling rate x=0.02 that Mo was entered in phase, has decreased transition temperature the reason of double exchange interaction weakens. But Mo doping in other samples has detected a impurity phase of CaMoO 4 and with increasing of Ca deficiency causedincreasing of T c in samples. According to the electrical resistivity measurements of with cooling rate samples was in agreement with the susceptibility data , but without cooling rate samples hade note regular behavior. For calculation of samples oxygen used iodometry titration technique and oxygen decreasing in two series of samples without doping calculated almost 0.05. Other samples Pr 0 . 5 Sr 0.5 MnO 3 were prepared by a