This paper reports a method for generating single crystals by using droplet based microfluidics. We studied theoretical background, and estimated the droplet size for obtaining only one ciystal within each droplet. We also investigated crystallization behavior of four model proteins to confirm the effect of protein molecular diffusion on crystallization. A single ciystal was obtained within a microdroplet which was smaller than the critical size. We consider that single crystallization or decoupling protein nucleation and crystal growth can be easily achieved by controlling droplet size.