A new micropump principle without mechanical valves is proposed. Flow rectification is achieved by a pair of dynamic valves, the pressure drop through each of which can be individually adjusted by controlling the liquid temperature in the valve channel, thus changing its viscosity. This method has a potential for miniaturization of complex liquid handling systems, since it allows bi- directional liquid transfer with a single micropump, by applying appropriate activation sequences for the valves and the pressure source. The necessary specification and the possible performance are predicted through FEM analysis of thermal and flow systems. By a preliminary experiment using a prototype pump structure fabricated with silicon based technology, the basic function of the valve elements has been confirmed.