Based on the strength theory of rock

a width reduction method for determining safety size of pillar is proposed in room and pillar mining. Through reducing pillar width gradually

the process of pillars from stable state to limit equilibrium state is studied; and the pillar critical width is obtained. Then the pillar safety width can be determined via introducing the pillar safety factor. By the width reduction method

the pillar safety width of Mofang mine No. 3 is studied to obtain the pillar safety widths in different spans. It is shown that the pillar safety width calculated by width reduction method is in good agreement with that by the Lunder theory through comparison; and the former is safer. According to the mining conditions of Mofang mine No. 3 and the results calculated by width reduction method

the pillar widths are determined to be from 5 m to 5.5 m

responding to the pillar span of 13 m. By monitoring the goafs overlaying strata of Mofang mine No. 3

it is found that the strata movement is relatively stable; the values of ground surface settlement are controlled in the range between 120 mm and 150 mm. To a certain extent

the fundamental parameters of room obtained by width reduction method are reasonable.