A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study single-phase liquid flow forced convection under uniform heat flux boundary condition. Standard micromachining techniques were utilized in the fabrication of the integrated microsystem. Utilizing a waferbond-and-etch-back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a 1.5μm thick composite membrane; thus, allowing experimentally good control of the thermal boundary conditions. A threedimensional numerical simulation model has been constructed to investigate the heat flux distribution. The results show that upstream the cold working fluid absorbs heat, while downstream the warmer working fluid releases heat. The Nusselt number is calculated based on the computations, which are compared with analytical and experimental results. The wall Nusselt number in a microchannel can only be estimated by conventional analytical solution in a limited Reynolds number range. The estimated Nusselt number for forced convection is found to be highly dependent on the location of the temperature measurements.