Abstract: Rock grain size parameter is the key parameter of reservoir rock physics analysis. The study of the relationship between NMR T₂ spectrum and rock grain size distribution curve showed NMR T₂ distribution directly related to and particle size distribution of rock. Hence NMR T₂ spectral data can be used to retrieve the size distribution of rock. Firstly, based on the core experiment results, the rock particle size distribution is calculated from the NMR T₂ distribution, using the conversion method of piecewise nonlinear calibration for each core, adjusting the calibration parameters to make the particle size distribution calculated approximate with the particle size distribution got thourgh core analysis, then obtain each core conversion scale parameter when error is minimum. Then classify the core and obtain scale parameters of each type according to the parameters of pore and permeability. At last, the granularity of down-hole rocks is inversed according to the actual NMR T₂ distribution curve. It turned out that the inversion results fitted most core analysis results, which suggested the method is reliable and can provide accurate, continuous rock size distribution profile for geological reservoir rock physics analysis.