For millimeter-sized working micromachines using fluid power with high density, a novel microvalve with simple structure using magneto-rheological (MR) fluid is proposed and called the micro MR valve. The micro MR valve utilizes the apparent viscosity controllability of the MR fluid in the magnetic field. To control sufficiently high magnetic flux density in the microvalve, a magnetic circuit using a permanent magnet, a thermosensitive ferrite, and Peltier elements called the thermocontrol magnetic circuit is introduced. First, the micro MR valve is proposed and the basic characteristics are theoretically investigated. Second, the thermocontrol magnetic circuit is fabricated and the static and dynamic characteristics are experimentally investigated and improved. Finally, a 3-port micro MR valve is fabricated and the characteristics are experimentally clarified and improved.