THE PERFORMANCE OF DIRECT EVAPORATIVE COOLER (DEC) AT VARIOUS OUTDOOR RELATIVE HUMIDITIES WAS NUMERICALLY STUDIED, AND THE GEOMETRIC AND PHYSICAL CHARACTERISTICS WERE OBTAINED BASED ON thermal comfort CRITERIA. FOR THIS, MATHEMATICAL MODEL BASED ON THE CONSERVATION EQUATIONS OF MASS, MOMENTUM AND ENERGY WAS DEVELOPED TO DETERMINE THE HEAT AND MASS TRANSFER CHARACTERISTICS OF THE SYSTEM. IT IS FOUND THAT DEC CAN PROVIDE thermal comfort CONDITION WHEN THE OUTDOOR AIR TEMPERATURE AND RELATIVE HUMIDITY (RH) ARE IN THE RANGE OF 29-41OC AND 10-60%, RESPECTIVELY. IN ADDITION, AS THE RH OF AMBIENT AIR RISES, SMALLER DIRECT COOLER IS NEEDED. FINALLY, A DESIGN GUIDELINE WAS PROPOSED TO DETERMINE THE SIZE OF REQUIRED PLATE HEAT EXCHANGERS AT VARIOUS OPERATING CONDITIONS.