Agglomeration kinetics of sodium chloride crystals in batch crystallization are examined from the changes of the number of suspended particles experimentally. Product purity was measured and the mechanism of purity decrease by agglomeration is discussed. From the changes in particle size distributions and the number of suspended particles, rapid agglomeration of NaCl crystals is found to occur immediately after seeding. The agglomeration kinetics were correlated with the number of the suspended particles and the solution supersaturation. The product purity is found to be lower when the average number of elementary crystals constituting agglomerates is larger. It also becomes lower when the number of seed crystals is larger and the initial supersaturation is higher.