IN THIS ARTICLE THE INFLUENCE OF MAJORS IN PRETREATMENT OF WASTEWATER SUCH AS AMOUNTS OF NAOH, COMPOSITION OF MIXTURE OF COAGULANTS AND MIXING RATE OF FIRST PRETREATMENT REACTOR ON THE AMOUNT OF TURBIDITY, TOTAL HARDNESS, CONTENT AND ELECTRICAL CONDUCTIVITY OF EFFLUENT WASTEWATER ARE INVESTIGATED EXPERIMENTALLY AND THE OPTIMUM VALUES ARE PRESENTED. EVENTUALLY THIS PAPER, SENSITIVITY ANALYZING SHOWS THE IMPORTANCE OF THESE FOUR MAJORS ON THE PERFORMANCE OF PRETREATMENT PROCESS.