The mechanism of mesoscale cyclogenesis in winter monsoon air streams is studied by data analyses of observations and linear instability analysis. Mesoscale cyclones, with a diameter of 200-700 km, occasionally develop off the west coast of Hokkaido, Japan. Based on the analysis of satellite images, the mesoscale cyclones are classified into two types according to their horizontal scale: Type I (200-300 km in diameter) and Type II (500-700 km). The observational studies suggest that the baroclinicity in the lower troposphere is important for the mesoscale cyclogenesis. The quasi-geostrophic equations are used to investigate an instability problem of a baroclinic flow with both the zonal and meridional components. The basic flow of the model expresses the essential elements of the observed wind profiles. The linear instability analysis showed that two unstable modes are found: Mode I with a wavelength of 200-300 km and Mode II with 500-700 km. The former is caused by the vertical shear of the meridional wind component and the latter mainly by that of the zonal wind component. The comparisons between theoretical and observational aspects indicate that Mode I and Mode II can account for the characteristic properties of Type I and Type II, respectively. The energetics shows that both Mode I and Mode II are maintained by the increase of the eddy available potential energy and its conversion into the eddy kinetic energy. Based on the data analyses of observations and the linear instability analysis, we concluded that the mesoscale cyclones are baroclinic instability disturbances developing in a particular baroclinic flow.