Microcantilever sensor with embedded piezoresistor has been proposed to measure the surface stress change from biochemical reaction. However, the sensor performance is adversely influenced by the piezoresistive thermal stress and biaxial surface stress loading. A mechanics model of piezoresistive microcantilever subject to surface stress loading is developed in this paper. A double-microcantilever design composed of the top immobilized microcantilever and the bottom sensing microcantilever is also proposed such that the surface stress loading can be converted to a concentrated force loading. The effect of biaxial surface stress can thus be limited to the immobilized microcantilever with the uniaxial strain in the sensing microcantilever. Analyses show that the surface stress sensitivity can be increased by high length ratio and lower thickness ratio of the two cantilevers. More than two orders of magnitude in measurement sensitivity can be achieved and the induced thermal noise can be minimized.