A field experiment was conducted on the Guanzhong Plain, China, to evaluate the effects of N and S application on characteristics of winter wheat (cv. Xiaoyan 22) grain filling and yield, using a central composite rotatable design with two factors nitrogen (N) and sulfur (S). The results showed that, with the combined application of N and S, grain filling followed a "S" curve which increased slowly at first, then quickly, and then slowly again. With 108 and 267 kg N x hm(-2), the grain fill duration, theoretical maximum grain yield, and average filling rate decreased as the S application rate increased. With 97.5 and 202.5 kg S x hm(-2), N application improved grain filling parameter values. With 187.5 kg N x hm(-2) or 150 kg S x hm(-2), the parameters firstly increased, reached a maximum, and then decreased as the S or N input increased. Grain filling rate increased for 25 days following anthesis, and then declined at a rate that varied among treatments. When the N input was > 187.5 kg x hm(-2) or the S input was > 150 kg x hm(-2), the grain filling rate decreased with increasing the S or N input. The results also indicated that combined application of N and S fertilizers at appropriate rates could increase the head density and grain yield. Using the regression equations, highest grain yields estimates (> or = 3753 kg x hm(-2)) were achieved with the combination of a high N rate (178.31-256.36 kg x hm(-2)) and a moderate S rate (131.95-167.65 kg x hm(-2)).