Abstarct: The longwall mining is widely used to exploit underground coal deposits in Iran. In this method, design of pillars, especially chain pillars, is one of the most important parts in the mining design. Pillars dimensions was conventionally determined considering some factors such as; coal compressive strength, pressure on pillars and ratio of pillar width to pillar height. Today, the conventional methods which are used to determine pillars dimension has been replaced by numerical methods. However, those approaches have a deterministic nature. Therefore, it is necessary to validate their results using data obtained by instrument or probabilistic methods. In this research, as the first stage, the dimension of coal pillars in Tabas coal mine (No.1) has been determine in all possible depths using empirical methods. In the second stage, pillars system including chain pillars, barrier pillars and adjacent panel are modeled. The model is then analyzed using FLAC3D software to find the side abutment and the front abutment pressure. baxsed on the dimensions of pillars reported in the basic designs of Tabas coal mine; the results of numerical analysis are shown that the chain pillars safety factor of is 1.6. In the last stage, by the probabilistic methods, first-order reliability moment (FORM) and Monte- Carlo Simulation (MCS), the results of the numerical model have been validated. baxsed on this validation methods, the probability of the pillars failure are approximately computed between 14-25 percent.