Nd0.5Sr0.5MnO3 nanoparticles with average sizes of 32-1000 nm in diameter were prepared by sol-gel method. The synthesized nanoparticles were characterized using X-ray diffraction, high- resolution transmission microscopy, scanning electron microscopy, and vibrating sample magnetometer. All samples have the single-phase orthorhombic structure, and the grain size increases with the increase in annealing temperature. A charge ordering (CO) transition at T(CO) and a ferromagnetic-paramagnetic transition at Tc were observed in 1000 nm nanoparticles. With the decrease in particle size, CO transition gradually shifts to lower temperature, becomes increasingly weak, and disappears for 85 nm nanoparticles. An inverse magnetocaloric effect with positive magnetic entropy change was observed around T(CO), and it decreases with the decrease in size due to the suppression of CO phase. The observed negative magnetic entropy change at Tc shows a surprising nonmonotonic behavior with the variation of particles size. All these results may give rise to a new insight into the magnetothermal behaviors in nanosized CO perovskite manganites.