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The influence of nitrogen (N) and sulphur (S) fertilizer on yield and quality of spring oats and the N and S metabolism of sheep was determined. Eighteen crossbred wether lambs (about 30 kg) were fed oats fertilized with N (0, 138 kg N/ha as of urea) and S (0, 30 and 60 kg S/ha as gypsum) assigned randomly to six treatments in an experiment of 2×3 factorial design. On S-deficient soils, S and N fertilization increased dry matter (DM) by 10.5 and 71.9%, and crude protein (CP) yield of oats by 11–13 and 94.5%, but there was no interaction between N and S fertilizer. S fertilization increased S content of oats from 0.14 to 0.29% and significantly reduced the N:S ratio from 14.4 to 7.22. There was a significant interaction between N and S on N content. N and S fertilization significantly increased apparent DM digestibility of oats by about 4%, but did not affect intake (g/kg BW0.75), DM intake being almost 60 g/kg BW0.75 for all treatments. N increased average daily gain of lambs by 62.4–78.6% if S was supplied, being twice the gain without S supply but no interaction between N and S was observed. N retention was affected by interaction between N and S fertilization. N retention and N utilization were positively correlated with S intake. N:S ratio in oats was positively correlated with N metabolism. S fertilization increased N utilization efficiency and vice versa. The critical levels of total S in oats were 0.20 and 0.23%, and the critical levels of N:S ratio were 17.0 and 15.7 for maximum DM and CP yield, respectively. The critical levels of S content and N:S ratio for maximum average daily gain of lambs were 0.26% and 10.5, respectively. Therefore, in the diagnosis of S status in soil–forage–animal systems, the use of total S and N:S ratio in the forage tops for the maximum CP yield is preferable to that for the maximum DM yield because the values are more closely related to the requirements for maximum daily gain of lambs; similarly, the use of a combination of total S and N:S ratio is preferable to the use of N:S ratio alone.