This paper presents the design and analysis of high sensitive diaphragm for electret microphone applications. In general, the sensitivity of electret microphone depends on the electrical sensitivity and the mechanical sensitivity of the diaphragm. The high mechanical sensitivity can be determined by the mechanical properties of the diaphragm. In order to obtain high sensitive microphones, the properties of low tensile stress and larger area of the diaphragms were fabricated. In this study, the spacer and the thickness of the backplate of electret microphone are 16 μm and 120 μm, respectively. The diaphragm made of polyimide and Si3N4 materials with various thickness and area were evaluated. It was found that the diaphragm made of polyimide presents the better sensitivity as compared with that made of Si3N4 materials. On the other hand, the larger area of diaphragm results in the higher sensitivity of the microphone. The corresponding sensitivity is 9 mV/Pa in the frequency range of 50 Hz to 12 kHz under the electret material with -120 V.