The thermal impedance of Modular Micro-channel Cooler (MCC) is the summation of the thermal impedances from heat source to silicon substrate, the silicon walls (fins), the interface between the PN junction bottom and indium layer, and the boundary layer of the coolant. The equations about the thermal conduction of MCC have been established and solved. The measurement of the thermal impedance of the MCC gave three curves that are the temperature rise versus the thermal power dissipated, the water flow versus the pressure across the device, and the wavelength versus the inverse water pressure. The calculated value of the thermal impedance from solving equations is in agreement with the measured value of the thermal impedance from the three curves.