Preprocessing is the essential technique in the implementation of the simplex method for large scale linear programming problems. In this research, we explained the effect of preprocessing in the simplex method, classified the techniques into four categories, and compared our results with those of HOPDM, CPLEX and Soplex by computational experiments. We also noted some implementing issues of preprocessing for the simplex method and the recovery of the optimal basis of the original problem from that of the preprocessed problem.