-The gas-liquid two-phase mass transfer process in microchannels is complicated due to the special dynamical characteristics. In this work, a novel method was explored to measure the liquid side volumetric mass transfer coefficient k L a. Pressure transducers were utilized to measure the pressure variation of upward gas-liquid two-phase flow in three vertical rectangular microchannels and the liquid side volumetric mass transfer coefficient k L a was calculated through the Pressure-Volume-Temperature correlation of the gas phase. Carbon dioxide-water, carbon dioxide-ethanol and carbon dioxide-n-propanol were used as working fluids, respectively. The dimensions of the microchannels were 40 μm×240 μm (depth×width), 100 μm×800 μm and 100 μm×2000 μm, respectively. Results showed that the channel diameter and the capillary number influence k L a remarkably and that the maximum value of k L a occurs in the annular flow regime. A new correlation of k L a was proposed based on the Sherwood number, Schmidt number and the capillary number. The predicted values of k L a agreed well with the experimental data.