The objective of this paper is to find out the optimum cell size for contact detection in the algorithm of DEM (Discrete Element Method) to improve the calculation speed. The computing time of contact detection was measured by using a cubic box, which contains particles, to investigate the effect of cell size. The Contact detection was inefficient when too fine grid cell or large one was introduced, the optimum cell size was given around 1.5-2.0 times of particle radius. The theoretical formula for the optimum cell size was proposed by considering the effects of the number of searched cells and contact checks. A significant correlation between the computing time or mm work and the theoretical equation was seen in all conditions of volume fraction. Furthermore, it was found that the optimum cell size was the size, which each cell has about 0.7-0.8 particles in it, and this result was applied to a modelling of random-sized particles. Therefore, the calculation speed of DEM would be improved by using the optimum cell size for the contact detection.