In this thesis, structural, magnetic and magnetocaloric properties of La 0 . 5 Ca 0 . 5- x Sr x MnO 3 )x=0, 0.2, 0.3, 0.4, 0.5) perovskite manganites have been studied. To study the structural properties of the samples, x-ray diffraction patterns along with their rietveld analysis have been used. It is found that the x=0 crystallizes in orthorhombic structure with Pnma space group, while the XRD patterns of x=0.2 and 0.3 can be refined well by a combination of two orthorhombic phases of space groups Pnma and Fmmm. For x=0.4, the best fit was obtained using a combination of two space groups Pnma and Imma. The end compound (x=0.5) crystallizes in tetragonal structure with space group I4/mcm. These results show that Sr substitution has a large effect on the crystal structure. Magnetic properties of the samples have been studied by measuring magnetization versus temperature and magnetic field. The magnetocaloric effect (change of the magnetic entropy) has been calculated by thermodynamic relations. The results show that the phase coexistence state can be investigated by magnetocaloric studies (especially the field dependence of magnetic entropy change at constant temperature). Magnetic entropy change (?S M ) shows positive (negative) peak at the vicinity of T N (T C ). However, in the intermediate temperatures between T N and T C , both the sign and magnitude of ?S M are strongly dependent on temperature and magnetic field which is a manifestation of the competition of FM and non-FM phases. This effect is more pronounced in the parent compound, La 0.5 Ca 0.5 MnO 3 , in which the intermediate phase separation ranges between 160 - 225 K. The substitution of Ca by Sr enhances the ferromagnetic state, weakens the phase separation and thus narrows the temperature range in which the field related effects (such as the sign change of ?S M are observed. The critical exponents, n, ?, ? and ? have been calculated. The critical exponents obtained for sample x=0.3 match with the values of the mean-field theory.