An interface-tracking method based on a phase-field modeling is evaluated through numerical simulation of thermal liquid-vapor flows with phase change in micro-fluidics devices. In the modeling, the interface with a finite width between phases is autonomously formed while satisfying the free-energy theory. The phase-field method therefore needs no conventional elaborating algorithms for advection and reconstruction of the interfaces. The numerical results of two-dimensional van-der-Waals fluid flows around a critical point prove that the method has a potential to capture liquid-vapor motions in the non-ideal fluid with heat and mass transfer across the interfaces, such as bubble nucleation on a heater and cavitating flows around a solid body.