The characteristics of the pressure drop of fluid were investigated in three different kinds of stainless steel microchannels with diameter of 0.56, 1.00, and 1.80 mm based on the absorption in the microchannels. For the single-phase flow pressure drop, nitrogen being used as the test fluid, it was found that the transition between the laminar and turbulent flow occurred at the Reynolds number of about 2000, which was different from some previous reports in which the early flow transition was observed in microchannels. For the two-phase flow pressure drop, an industrial chemical absorbent (40 wt % MDEA solution) and nitrogen being taken as the working fluid, it was found that the existing correlations including the homogeneous flow model and separated flow model failed to predict the experimental results because of the special operating condition in which the ratio of volumetric flow rate of gas to liquid phase ranged from 100 to 3400. A new correlation to predict the two-phase flow pressure drop was developed in the form of the Lockhart-Martinelli type which considered the effect of surface tension, gas phase inertial force, Reynolds number of liquid phase, and the Martinelli parameter. The predicted data with new correlation showed good agreement with the experimental results.