Abstract: A new method,transmission spectrum method combined the normal simplex algorithm,is proposed to measure the grating parameters.Firstly,the transmission spectrum of the range from 200nm~900nm is obtained according to a certain grating model with the Fourier model method and reflection and tran smission coefficient matrix method (RTCM),based on which the Gaussian noises with different standard deviations are added to it to get the simulated measuring value of transmission spectrum,then five points are gotten respectively from the transmission spectrum with or without Gaussian noises.Secondly,the original supposed value of grating parameters is brought to the estimating function.Through repeatedly computing with the normal simplex algorithm,the grating parameters are obtained.