This paper presents a synthesis method and its experimental validation for microfluidic concentration gradient generators that use multi-stream laminar flow. The method is based on an iterative framework in which designs are evaluated with an analytical convection-diffusion model. Comparing to the expensive trial-and-error experimentation or the time consuming finite element simulation, this design scheme enables the efficient design of concentration gradient generators capable of generating complex concentration profiles. We describe the synthesis method, and validate it by experiments with devices that are designed based on the method. The concentration profiles obtained from the experiments agree well with the prescribed ones.