We measured the specific heat (C) of Gd1−xErxNiSi. The values of C for x=0 (GdNiSi) peaked at 11.5 K due to the antiferromagnetic order. The Néel temperature and the values of the entropy of Gd1−xErxNiSi decreased with increasing Er content up to x=1 (ErNiSi). The end member ErNiSi showed two successive transitions at 2.8 K and 2.3 K. The values of C at a temperature at which a peak appeared decreased with an increase in x, except for x=1. These results originate from the reduction of the multiplicity of the crystalline electric field ground state and the atomic randomness due to the Er substitution.