The crystallization of struvite-K (MgKPO4 center dot 6H(2)O) can simultaneously recover phosphate and potassium from source -separated urine. However, its crystal size has been rarely reported despite significantly affecting the crystallizer performance. Experimental results show that the average size of struvite-K was 15-63 mu m in ammonium-depleted synthetic urine under the examined conditions with a stirred reactor. A low supersaturation ratio (Sr) or the same Sr induced by low pH and high Mg:P ratio resulted in large crystal size. Large crystals also formed at high agitation speed, small feed volume, or feeding close to the stirring impeller. The size distribution of struvite-K crystals was predicted using the coupling resolution of the population balance equation to on site turbidity measurement. The estimated kinetic parameters quantitatively described the nucleation and crystal growth of struvite-K under different conditions of supersaturation, feeding, and mixing. This work proposes approaches to obtain large crystals while reducing fines of struvite-K.