When using the phase mask technique to write fiber Bragg gratings(FBG),we observed that the Bragg wavelengths monitored were different while the fiber was held with different strain,by which the strain optic effect in FBG was directly detected.Through the analysis and calculation of the test data,we found that the effective strain optic coefficient of the fiber is not constant as commonly believed,but increases slightly as the axial strain of the fiber increases.