Based on the Mach-Zehnder interference effect between the core mode and the cladding modes, the interference fringes are formed by a pair of cascaded long-period fiber gratings (CLPFG). It is found that the spectral spacing and the wavelength of these fringes are related to the absolute value of waveguide dispersion factor of LPFG. So by measuring the characteristics of the transmission spectra of CLPFG, the absolute value of waveguide dispersion factor γ can be deduced. At the same time, the thermo-optic coefficient of effective refractive index μ can also be deduced by measured the thermo-effect of these fringes. By this method, at the wavelength of 1554 nm, the values of γ of HE14 cladding mode and μ are calculated to be 0.874 and 4.08×10-5℃-1, respectively.