Abstract: In practice of homogenization with Monte Carlo method,critical effect is studied and B_N theory is applied taking critical effect into account.Fundamental mode with buckling B is used as a measure of finite size.A critical spectrum in solution of fine-group Bl equations is used to correct weighted spectrum for homogenization.A PWR prototype core is examined to verify that the method generates few group constants effectively.In addition,a zero power physical experiment verification is performed.Numerical validations show that B_N theory is adequate for critical correction in multi-group constants generation via Monte Carlo method.