By using chemical metallization, nickel layer is coated on the fiber Bragg grating with different thicknesses. The inner stress occurs due to different thermal expanding coefficients between nickel layer and fiber grating layer, which is influenced by metal-film thickness. In this paper, the relationship between the wavelength shift and the thickness of nickel layer is theoretically analyzed. Moreover, the in-situ observation of the fiber grating during electroless plating and the wavelength shift as a function of metal-film thickness are investigated. A good agreement is found between the experimental and calculated results.