Effects of SiO2 additive on sodium perborate tetrahydrate crystallization (via chemical reaction of NaBO2 and H2O2) has been studied in a MSMPR-type reaction crystallizer. CSD and microscopic observations are used to evaluate the secondary nucleation behaviour. Secondary nucleation rate decreases in case of SiO2 addition of up to 200 ppm. Higher SiO2 content results in higher primary nucleation rates. Abrasion resistance increases with the amount of SiO2 additive.