A giant magnetocaloric effect has been observed in NdMn2Ge0.4Si1.6 associated with the first-order magnetic phase transition from antiferromagnetism to ferromagnetism around T-C=36 K. The magnetic entropy change -Delta S-M and adiabatic temperature change Delta T-ad have been determined from magnetization and specific heat measurements (B=0-5 T) with -Delta S-M calculated by the Maxwell relation and Clausius-Clapeyron method. The values -Delta S-M(max) = 12.3 J kg(-1) K-1 and refrigerant capacity similar to 95 J/kg for Delta B=0-2 T as derived from the Maxwell relation, together with the small hysteresis (thermal