Abstract: Based on pressured-mercury testing and fractal theory, studying on pore size classification, pore development characteristics and pore fractal dimension. The results showed that the pore system of Longkou lignite was divided into three parts. The pore with a radius over 1028nm was macro pore; the pore with a radius less than 72nm was micro pore; and the pore with a radius of 72~1028nm was transition pore. The porosity ranged from 10% to 15%, and the mean was 12.669%. The specific pore volume ranged from 0.09ml/g~0.13ml/g, and the mean was 0.115ml/g. The specific surface area was between 39m2/g and 70m2/g, and the mean was 56.529m2/g. The pore development of coal sample was great, and pore volume and surface area were large. The pore volume of micro pore was more than 80% of the total capacity, and micro pore was spread more evenly. The fractal characteristics of macro pore and transition pore was very good and the average fractal dimension was 2.56, which showed that this part of pore had greater pore structure and permeability was better.