The effect of nitrogen-energy balance on associative effects of feedstuffs was investigated using in vitro gas test. Ammoniated rice straw diet was supplemented with 40% of mixture of rapeseed meal and maize in which their ratio was 25, 50, 75 and 100%, respectively, so that different ratio of rumen degradable nitrogen to digested organic matter were obtained. The associative effect was defined as the difference between the observed digestibility and the predicted value from individual feed fermented alone. The associative effect values ranged from 4 to 9%, varied with ratio of rapeseed meal to maize. The greatest response was pronounced when the ratio was 75%, at which the microbial protein synthesis was also maximum. It is inferred that the associative effect is influenced by the nitrogen-energy balance.