Lately, LIDAR technique has been widely used to generate digital terrain model of forest areas.Because,it is not possible to generate digital terrain model in forest areas by satellite and aerial images and also by land surveyor,economicaliy is not possible because of high expenses and taking a long time.The method which is explained in this report for extracting non-terrestrial points from last pulse points in primary step is skewness balancing algorithm.This method is almost capable of removing non-terrestrial points from last pulse points and the remain points can be considered as terrestrial ones.Using this algorithm,most of non-terrestrial points were removed but since the removed points were extracted from last pulse points,it might contained some terrestrial points.For obtaining these points,the points due to primary step must be substracted from last pulse points and then compare the results,with first pulse points by elevation differences.Then this result should be combined with the primary step points to gain new terrestrial points.But because of the ground slope,non-terrestrial points near ground could not be removed by two steps before.For removing them,the slope filter method can be used.The remained points can be accepted as terrestrial points.At last,by using B-Spline method,these points will be interpolated and we get digital terrain model ,with the height accuracy of .2 meter.