With the development of miniaturization of devices in various fields (medicine, chemistry, microelectronics), a central problem is the study of hydrodynamic processes in microsystems. One of the main parameters characterizing a two-phase flow is void fraction. In the present work, the void fraction in the slit microchannel with the height of 56 μm and the width of 10 mm was determined using shlieren technique and special algorithm developed in Matlab®. Furthermore, the dependence between time averaged void fraction and homogeneous void fraction was created. The comparison between experimental data and existed void fraction correlations was performed. It was shown that correlations of Kawahara and Xiong and Chung developed for circle and rectangular microchannels qualitatively describe the behavior of void fraction in slit microchannel too.