The effect of seed load ratio on the growth kinetics of Mono-ammonium phosphate (MAP) under isothermal batch crystallization was investigated quantitatively. A direct parameter estimation method was proposed and applied to extract the growth kinetic parameters from a simple crystallization model using our experimental solution concentration decline data. The method assured the globally best parameters to be obtained and was found less sensitive to experimental errors. The linear growth constants kg and the growth order g were found to be in the range of 1,000-2,600 μm·min.1 and 0.93-1.12, respectively, for MAP crystallized at 40 oC. Both parameters decreased significantly with increase of seed load ratio and kg even showed a strong linear decline trend. The effective crystallization time also decreased with the seed mass. The proposed methodology could be extended to study the effect of other operation variables such as temperature and initial supersaturation on the crystal growth rate.