Abstract: In order to study the gas content distribution features of the Jurassic low rank seam in Ordos Basin and the evaluation significances of the coal bed methan e resources,a mathematical statistics and system theory method was applied to a study on the coal bed methane potential and the composition features,prediction of free g as in the coal and the calculation of the coal bed methane resource volume.The results showed that the coal bed methane potential in Ordos Basin was in middle grade of t he national low rank coal bed methane potential,the desorption gas content was 0.11 ~ 4.33 m3/ t,the lost gas was about 20% of the total gas content,the residual gas wa S less than5% generally,the free gas was 11. 96% ~ 48. 16% of the total gas content,the percentage of the free gas was related to the coal porosity and preservation condit ions. Based on a unit divided from the coal system as the unit of the coal bed methane evaluation,the total volume of the coal bend methane resource calculated with the V olume method was 75 833.62 x 108m3.