Twenty five surface soil samples taken from different sites and 36 soil samples from 6 soils, each consisting of 6 layers, were used to study the relations of the mineralizable N to the organic matter and total N. The mineralizable N was estimated by an aerobic incubation. The results showed that the mineralized N was very well correlated with the organic matter and total N for all the soils. However, the ratios of mineralized N to the organic matter or to the total N were remarkably different, and the mineralized N was more correlated with the ratios. These results strongly demonstrated that the amounts of the mineralized N were dependant on both the contents and the potentially mineralizable proport ions of the total N and organic matter, and that the contents of the total N or organic matter were unable to provide the information on the proportion made available to plants. Field experiments were conducted using 15 soils containing low accumulated nitrate N in soil profile, and the results revealed that the mineralizable N had good correlation with either wheat yield or N uptake. However, the organic matter or total N was not so as the mineralizable N, though the total N had a high correlation coefficient with wheat yield in case without P addition. The result further confirmed that determination of mineralizable N had its significance, and could not be replaced by determination of organic matter or total N.