Lanthanum doped Strontium manganites La0.25Sr0.75MnO3 perovskite nanocrystallites were synthesized by combustion method followed by annealed at the temperatures 600, 800, 1000 and 1200C for 6 hours. These nanocrystallites were analyzed using X ray diffractometer and UV -Vis spectrometer. From the XRD patterns, it was confirmed the formation of Rhombohedral structure (space group R c) without any impurity peaks. The average crystallite size was seen to increase gradually as function of calcination temperature from 13.6 to 22.7nm. From UV-Vis spectra ,the calculated optical energy gap of La0.25Sr0.75MnO3 nanocrystalline as reduced from 3.86 to 2.45 eV with the annealed temperature, reveals that the La0.25Sr0.75MnO3 nanocrystallites are semiconductors in nature. Hence La0.25Sr0.75MnO3 nanocrystallites annealed at different temperature showed novel structural, and optical properties.