This research presents a study of the design and constructionof afertilizer rotary dryerin a pig farm which usedwaste heat at 400 degree Celsius from an electrical generator. The designed rotary dryer consisted of two pipes with lengths of 10 meters. These pipes consisted of a drying and a cooling pipe, each with a diameter of 0.5 meter. The power was supplied by two 5 horse power motors, and 2 horse power blowers were used to suck in hot and cool air for drying.The fertilizer,with an initial moisture content of 55-60 %db,was examined. Three parameters were varied to study the optimal condition of drying. These were rotary speed, degree of tilt angle and air flow rate. The rotary speed was varied at 10, 15 and 20 RPM, the degree of tilt angle was set at 1, 3 and 5 degrees, and the air flow rate was varied at 60, 120 and 180 cubic meters per hour. From the experiment, the result revealed that the condition of 1 degree of tilt angle, 10 rpm of rotary speed and an air flow rate of 180 cubic meters per hours were the optimal conditions for drying the fertilizer. The moisturein the fertilizer was reduced from 57.84 %dbto 12.45 %db. The drying rate was 56.93 kg/h, and the thermal efficiency was 79.24 %. The energy consumption was 328.64 kWh/ton. The capacity of the designed rotary dryer was 3 tons/day which is three times more than the amount of the traditional drying method. If the cost of the rotary dryer is 450,000 Baht, the payback period will be 3.73 months.