In this research, sludge characterization and rates of mineralization was evaluated in different sizes of rainbow trout (Oncorhynchus mykiss) in the culture systems. Four weight groups of fish (20±3; 50±5; 100±9; 200±10 gr) in triplicates were introduced and cultured in the experimental units (200 liter tanks) for a three day period. The fish were fed with a commercial dry feed pellete (Faradaneh Co.). The rates of mineralization due to the effect of natural flora (microorganisms) of bacteria on the produced sludge, was detected. In this respect, the fish captured and remainder sludge and the rate of macro and microelements recorded in all the treatments during a 7 day period. Concentration of TAN (Total Ammonia Nitrogen) and NO3-, shwoed significant differences among all treatments within initial day until the third day (p<0.05). Concentration of total phosphorous (TP) increased and showed significant differences among treatments after the first stage of the experiment (three days) (p<0.05). EC and total hardness showed significant differences among the treatments after the first stage of the experiment as well (p<0.05). It was concluded that, bacterial mineralization of sludge and organic matter had larger effect at the first three days, in comparison with after harvest of fishes as well.