The formation of scales that is caused by calcium salt in heat exchanger or cooling tower proses is lead to serious problems such as pollution and decreasing of heat transfer. In this study the effect of crosslinked polymers on the crystallization of calcium oxalate has been investigated in aqueous solutions. Vinylphosphonic acid (VPA) and Ethylene glycol methacrylate phosphate (EGMP) with different NN-Methylene bis acrylamide (MBA) contents have been synthesized for using as additives. The effect of the polymer on the rate of crystallization of calcium oxalate have been determined by recording the decrease in [Ca2+] as a function of time in a solution. The ability of polymers to act as inhibitor was evaluated by R0/R ratios which shows the ratio of the rate of crystallization of the pure solution (R0 mol/L min) to the rate of crystallization in the presence of additive (R mol/L min) at the same concentration and temperature. The higher R0/R values correspond to a better inhibition. Since additives have great influence on crystal size and shape we investigated the selective effect of the polymer on the crystal morphology and size using SEM. The inhibition of calcium oxalate crystallization by crosslinked polymers decreased with increasing NN-Methylene bis acrylamide content. The results show that VPA is a more effective inhibitor than EGMP.Keywords: Calcium oxalate ethylene glycol methacrylate phosphate vinylphosphonic acid crystallization additives.