The performance of the anaerobic filter has been tested on variety types of industrial as well as municipal wastes. This study aims to evaluate the different operational parameters of up-flow anaerobic filter packed with gravel media treating wastewater, at hot climate conditions. Design and performance parameters such as media size, hydraulic retention time and operation mode have been evaluated. The experimental work utilized three up-flow anaerobic filters packed with gravel media, which is considered as low cost option. A pilot scale model have been constructed and fed with primary treated wastewater from Al Aslogy wastewater treatment plant, Zagazig, Egypt. The filters were examined in parallel at different flow rates in order to evaluate the performance of UAF at ambient temperature as well as to determine the effect of media size. The effect of multistage operation also was determined. The pilot scale reactors show good removal ratios of COD, BOD5 and suspended solids. The average COD removal varied from 40 to 77 % and the BOD5 removal varied from 48 to 91 % depending upon the hydraulic retention time and mode of operation. The results show also that the media size has minor effect on the removal ratios. Multi stages operation enhanced the efficiency of the filter rather than single stage.