Microfluidic mixing plays a key role in various applications including biomedicine and chemical en-gineering. Conventionally, bright-field and fluorescence microscopy have been used for analysing microfluid-ic mixing. However, quantification of microfluidic mixing has remained challenging, mainly due to the lim-ited access for quantitative information from intensity images of conventional imaging techniques. Here, we present a method for quantifying transparent microfluidic mixing using a quantitative phase imaging tech-nique. Measuring optical phase delay maps in the microfluidic channels from the quantitative phase imaging technique yields quantitative fluid concentration maps along the channels. We show the quantitative phase imaging of microfluidic mixing in various types of PDMS microfluidic channels with different geometry, and the feasibility of the present method was validated by comparing with a result obtained by theoretical calculation based on Fick’s law.