A method of measuring a critical dimension comprises determining matrix information on a hypothesized parameter distribution on the critical dimension; obtaining a measurement spectrum for each wavelength of a light reflected from the three-dimensional structure; sampling a first candidate parameter vector for determining a numerical simulation spectrum that approximates the measurement spectrum; linearly transforming the first candidate parameter vector by using the matrix information; determining a first candidate parameter which minimizes a difference between the measurement spectrum and the numerical simulation spectrum within the linearly transformed first candidate parameter vector; determining a second candidate parameter vector derived from the first candidate parameter and having the hypothesized parameter distribution by using a heuristic algorithm; determining a second candidate parameter that minimizes a difference between the measurement spectrum and the numerical simulation spectrum within the second candidate parameter vector; and updating the first candidate parameter by using the second candidate parameter.