The conversion reaction of zeolite A into zeolite HS was studied at various reaction temperatures and alkalinities. The effect of crystal size of zeolite A on the rate of the conversion reaction was also studied. The rates of conversion increased at elevated reaction temperature and alkalinity and with small size of zeolite A crystal. The changes of zeolite content during the conversion reaction expressed as the equation（Z^1/3=K_appt） based on the surface reaction. The crystal size of zeolite HS produced did not show any relationship with that of zeolite A. The rate determinig step of the conversion reaction is the dissolution step of zeolite A and the growth of zeolite HS proceeded at the expense of soluted reactant occuring in liquid phase.