Since the invention and application of nitrogen (N) fertilizer, people always wanted to measure the effects of N fertilizer application by nitrogen use efficiency (NUE). The traditional NUE is the percentage of fertilizer N uptake by crop to N fertilizer rate, which didn’t consider the replenishing effect of fertilizer N to soil N consumption. Due to the defects of the concept and calculation, and the poor interpretations and understands of the results, there are a lot of misunderstands in the literature and daily communication. Therefore, many improved method for calculating NUE were proposed by researchers. However, although these methods had been involved the residual effects of N fertilizer on succeeding crops, they didn’t touch the core issue of the replenishing effect of fertilizer N to soil N consumption. Based on the main N flows in the soil-crop system and the relationships between the fertilizer N, soil N and crop N uptake (called three N), I proposed the concept and calculation of nitrogen fertilizer availability ratio (NFAR) in this study. The core item for NFAR is that the residual fertilizer N is regard as the replenishing to soil N consumption. I recognize that the NFAR is 50%~60% and the loss rate of N fertilizer is 40%~50% under current N management practices in China based on the analysis the data from the 15N tracer field trial, which reflects the high loss of fertilizer N in practices. It is possible to increase NFAR to 70%~90% by improved fertilizer N and agronomic managements. The NFAR expand the idea for the effects of N fertilizer application. It would be important for demonstrating the real effects of crop N uptake and soil N fertility maintain by N fertilizer, and would reflect real loss to environments of N fertilizer application.