To design microfluidic device, it is essential to predict flow properties in micro channel. However, microfluidics includes important but different effects comparing with conventional fluid mechanics. And because most microfluidic system have used rectangular channel which is not circular channel, Poiseuille number cannot be ignored. Our work explains the method of modeling and designing of microfluidic system considering Poiseuille number (Po number). Our modeling method can easily anticipate the characteristics of arbitrary shape channel flow at very low Reynolds number and can control mass flow rate accurately.