The Internal Stress and Young's modulus changes of LPCVD silicon nitride thin film by ion implantation were measured. The load-deflection measurement technique of the square membrane with size of 2mm×2mm was used. The tensile internal stress σ was reduced from 1.2 GPa to less than 0.1GPa and the Young's modulus E was reduced from 320 GPa to 260 Gpa. The effects of σ and E of a pressure sensor diaphragm, with a side of 2a and a thickness of t, on pressure sensitivity were calculated. In the case of (t/a)2_??_σ/E, lowering of sensitivity due to σ can not be neglected. Therefore, it appears that the sensitivity of micro-pressure sensor with silicon nitride film as the construction material for diaphragm can be improved by several times by utilizing the ion implantation technique.