Physical, chemical and microbial colonization of rice straw compost heaps were investigated during composting cycles. The temperature of all compost heaps highly increased, reached its maximum values at 20 days (29° C at 50cm depth). and gradually decreased to the ordinary temperature at maturity (80 days). pH values turned to acidity in the first few days and increased to neutrality at compost maturity. EC, Bulk density and nitrogen content were increased whereas, total organic matter, organic carbon and C/N ratio drastically decreased with developing of composting heaps. The total numbers of mesophilic bacteria, fungi and actinomycetes highly increased in the initial and final cycles with all compost heaps. Whereas, they decreased in the heating cycles (20 and 40 days). On the other hand, thermophilic organisms highly increased in the heating cycles. Bacteria was the most dominant microbes in the cooling and heating cycles followed by actinomycets and fungi. The changes in the microbial populations in all compost heaps depend mainly on the changes in compost temperature as a result of microbial activity and subsequent changes in physical and chemical parameters.