An improvement in the resonant microcantilever-based sensor's sensitivity is proposed by varying the microcantilever weight distribution. The relationship between the resonant frequency of different microcantilevers and their dimensions is investigated using the analytical method and finite element method. The principle to increase the resonant frequency drift when the target gas is absorbed by the sensor is established. The ratio of resonant frequency to cantilever mass is used to evaluate the efficiency of this principle. It is found that making the free end of the microcantilever thinner and narrower can enhance the sensor's measurement sensitivity, which is validated by the investigation of many different shaped cantilevers. Using these results, a dual-triangular microcantilever that has the highest frequency drift is proposed for sensing applications. The microcantilever's fabrication is also simply discussed in this paper.