Thermoelectric properties of BiSbTe-based composites dispersed with a small amount (<1 vol%) of amorphous Si₃N₄ (a-Si₃N₄) nanoparticles (∼25 nm) were investigated in the temperature range from 303 K to 483 K. The results indicate that with a-Si₃N₄ content increasing, the thermopower (S) of the a-Si₃N₄/BiSbTe composites increases substantially at T < ∼370 K, due to the decreased carrier concentrations and the enhanced energy-dependent scattering of the carrier at the heterojunction potential. Simultaneously, a-Si₃N₄ nanodispersion causes ∼20–30% reduction in thermal conductivity (κ) owing to phonon scattering of nanoparticles as well as phase boundaries. As a result, high dimensionless figure of merit (ZT) values of up to 1.20 (∼303 K) and 1.38 (∼383 K) are obtained in Bi_0.4Sb_1.6Te₃ incorporated with only 0.44 vol% a-Si₃N₄ nanoparticles, demonstrating that the thermoelectric performance of the BiSbTe alloy can be improved effectively through incorporation of a-Si₃N₄ nanoparticles.