Abstarct: The long-wall mining method is one of the underground coal mining methods that used generally in low to medium slope laxyered deposits. In this method, Pillars are placed on both sides of the extraction zone in order to prevent subsidence at the ground level that protected building and plants. In this method, coal production, recovery and mining operations safety is highly depends on coal pillars design. With development of the long-wall mining method, analytical methods and numerical method could be useful for determining of pillar dimension. Pillar dimensions is very important due to safety and economic issue. Limitations in Traditional and Experimental Design Methods gives opportunity to Numerical methods for pillar design. By using numerical methods, it is possible to consider some factors, such as the sequence of extraction and the effect of confinement stresses that is the advantage of this method to analytical methods. In this research, according to the long-wall design report of TAZAREH coal mines, laxyer K8 has been selected for further studies. According to this report, the pillar dimensions considered 6×6 square meters. A Numerical model for pillar stability analyze performed in FLAC3D. To validate the numerical model, due to the lack of sufficient information, the existing graphs and empirical relationships have been used to analyze the stress on the foundations. According to the studies, the most critical pillar state occurs when the vertical stress reach at maximum amount, the maximum amount differ for upper and lower pillars. According to the results, for the pillar with a width of 6 meter, a critical state occurs for the upper part when the distance between coal face and pillars reach to 60 meters and it happen for the lower part when this distance reach to 120 meters. For further analysis to reach optimum pillar width, models with a pillar width of 5, 4 and 3 meters has been made. From the results of these models, it can be concluded that the optimal width of the pillar is 4 meters in the upper part and 5 meters in the lower part. These results showed that with smaller dimensions of the pillars the recovery rate of coal will be increased