The dynamic magnetic properties of cluster glass in La0.25Ca0.75MnO3 nanoparticles with average particle size range from 40 to 1000 nm have been investigated by measuring the frequency and dc magnetic field (H) dependencies of the ac susceptibility. The frequency-dependent Tf, the freezing temperature of the ferromagnetic clusters determined by the peak in the real part of the ac susceptibility χ′ versus T curve with H=0, is fit to a power law. The relaxation time constant Τ0 decreases as the particle size increases from 40 to 350 nm, which indicates the decrease in the size of the clusters at the surface of the nanoparticle. The relationship between H and Tf(H) deviates from the De Almeida–Thouless-type phase boundary at relatively high fields for the samples with size range from 40 to 350 nm. Moreover, for the samples with particle sizes of 40 and 100 nm, Τ0 increases with increasing H, which indicates the increasing cluster size and may be ascribed to the competition between the influence of H and the local anisotropy field in the shell spins. All these results may give rise to a new insight into the behaviors of the cluster glass state in the nanosized antiferromagnetic charge-ordered perovskite manganites.