The influence of fertilization with urea and ryegrass on nitrogen utilization and transformation in red soil has been studied by using ¹⁵N tracer method. When urea and ryegrass were applied alone or in combination, the percentage of N uptaken by ryegrass from labelled urea was 3 and 1.7 times that from labelled ryegrass for the application rate of 200mgN•kg^-1 and 100mgN-kg^-1, respectively; combining application of ryegrass and urea reduced uptake of urea N and increased uptake of ryegrass N by ryegrass plant, but the percentage of N residued in soil increased for urea and decreased for ryegrass; when urea and ryegrass were applied alone, the percentage of N residued in soil from labelled ryegrass was more than 69% while that from labelled urea was less than 25%, and much more ryegrass N was incorporated into humus than urea N.