The pot experiments were carried out to investigate the availity of the residual N from rabbit excreta and ammonium sulfate and its relationsliip with the residual N components by using the isotopic ¹⁵N tracer technique. The results indicated that the residual effect of fertilizer N was low. The residual N from rabbu excreta utilized by wheat plant was 3.6-4.8%. Most of the residual N (82-89%) was left in the soil and 7-14% of the residual N was lost; while the corresponding values for the residual N from ammonium sulfate were 3.6-5.3%, 69-76% and 19-28% respectively. The amounts of amino acid-N and acid hydrolyzed-unidentified N fractions from these two fertilizers were relatively high. However, the residual N from ammonium sulfate in these two fractions was much more than that from rabbit excreta. Ninety-five percent of the mineralized N from the ammonium sulfate left in the soil came from these two N components while that for rabbit excreta was only 66-84%.In the paper the signific;ince and application of the ¹⁵N% dff value are also discussed.