The study of magnetization on beta-FeSi2 nanoparticles with particle size of 15 nm, 22 nm and 75 nm confirmed that semiconducting beta-FeSi2 became superparamagnetic as it was reduced into nanoparticles. The superparamagnetic characteristics were identified by the irreversible magnetization and blocking temperatures of FC and ZFC measurements. The correlations among blocking temperature, anisotropic constant and particle size were studied, the enhancement of anisotropic constant with size decrease was conjectured to be attributed to the size effects.