The crystal size distribution (CSD) and crystal purity of potassium chloride (KCl) have been measured in an MSMPR crystallizer by using two different impellers. A small amount of sodium chloride (NaCl) was added as an impurity tracer in order to measure the inclusion ratio of mother liquor inside KCl crystals. The effects of mixing, residence time, growth rate and crystal size of KCl on the inclusion of mother liquor were investigated. The crystal growth rate and nucleation rate were determined from the population density balance theory. The attrition of crystals changed round shape of the crystal and affected significantly the inclusion of mother liquor in the crystals. The roundness of KCl crystals was proposed as a new factor to take the attrition effects into consideration. An empirical relationship between the inclusion of mother liquor and the operating variables was obtained.