The effect of anionic polyelectrolytes on the crystallization kinetics and some important industrial properties of sodium perborate tetrahydrate, such as particle size, bulk density and degree of attrition were studied by using an MSMPR type laboratory crystallizer. The anionic polyelectrolytes having similar chemical structures but different charge density were used in the experiments. The experimental results showed that the anionic polyelectrolyte, which has 30% degree of anionicity does not affect the growth and nucleation rates of sodium perborate tetrahydrate compared with the sodium perborate tetrahydrate crystallization in the absence of polyelectrolyte. Decreasing the nucleation rate and increasing the growth rate in the presence of the anionic polyelectrolytes having low anionicity (2 and 10%) and high anionicity (50%) caused an increase of the average particle size and the bulk density and the decrease of the degree of attrition of sodium perborate tetrahydrate crystals. It was seen that the anionic polyelectrolytes having high molecular weight affected the sodium perborate crystallization more than the anionic polyelectrolytes having low molecular weight.