magnetoresistance (LFMR) of La 0.8 Sr 0.2 MnO 3 /x MnFe 2 O 4 samples with x=0~10% (weight percents) have been investigated. The La 0.8 Sr 0.2 MnO 3 sample has been synthesized by thermal decomposition of acetates in citric acid (new method) and MnFe 2 O 4 has been synthesized by sol-gel method. Structural properties of all samples has been investigated by X-ray diffraction and results show that all samples have rhombohedral structure.Electrical resistance measurements of samples show that metal-insulator transition decreases by the increase of MnFe 2 O 4 level. This decrease is due to the grain boundaries effects. Also, the value of magnetoresistance in 77 K temperature increases for low doping level of 0 x 7.5 and then decreases at higher doping level, x=10. The spin dependent tunneling and scattering at interfaces of the grain boundaries are responsible for the increasing of LFMR at x 7.5, while reduction of LFMR at x=10 originates from the increase of the thickness of grain boundaries. In the second part, the influence of grain size on the structural, magnetic, electrical properties and magnetoresistance of La 0.9 Ba 0.1 MnO 3 samples has been studied. The samples have been synthesized by thermal decomposition of nitrates in citric acid and have been sintered at 800?C and 1350 ?C. The results of X-ray diffraction show that the width of picks decreases and diffraction intensity increases by increase of grain size. The lattice parameters and unit cell volume of samples have been obtained by FULLPROF soft ware, rietveld refinement. It shows that samples have rhombohedral structure and belongs toR-3C space group. SEM images show that grain sizes increase by increasing of sintering temperature. The grain size of samples that were sintered in 800 ?C and 1350 ?C In this thesis structural, magnetic, electrical properties and low field are in the range of nanometer andmicrometerrespectivelyAc magnetic susceptibility measurements show that all samples have paramagnetic-ferromagnetic transition and the magnitude of susceptibility increases by increase of grain size. This is due to decrease of interface distortion with the increase of grains size. Also the cluster spin glass is formed in the 1350 ?C sample. The values of electrical resistance at high temperature decrease and metal-insulator transition temperature increases by increasing grain size. This is also due to decrease of grain boundaries. For nanosize sample, insulator-metal transition temperature is lower than ferromagnetic-paramagnetic transition temperature. Also, magnetoresistance measurements of samples versus temperature show that LFMR decreases and CMR increases by increasing of grain size. Key words : Manganite, Nano crystal