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In order to profit from recycling sewage sludge through the soil‐plant system, it is necessary to know the amount of mineralizable organic‐N from sludge. The purpose of this study was to determine N‐mineralization of two sewage sludges in two different soils, comparing leached and nonleached incubation procedures. The cumulative N mineralized during successive incubation periods increased linearly with incubation time and sludge incorporation rate. The mineralization process was more influenced by soil type than by rate and kind of sludge applied. The amount of mineralized‐N was higher for the leaching procedure. This cumulative‐N expressed as the percentage of applied organic‐N was inversely dependent on sewage sludge rate added for the leached procedure and is independent of the rate for the nonleached. The N‐mineralization rate was 0.0202 ± 0.0011 and 0.0650 ± 0.0068 d−1, respectively, for leached and nonleached procedures. The potentially mineralizable N increased with the sludge rate applied and was higher for aerobic sludge and neutral soil. In general, the leached method gave twofold higher vlaues than the nonleached method. The net percentage of potentially mineralizable N vs. organic‐N added was 43.0 ± 7.8 and 27.7 ± 4.0, respectively, for leached and nonleached procedures.