For the investigation of the geometric nonlinear effect of suspension bridges under random wind loads, a frequency domain method based on a stochastic linearization technique is presented. The equation of motion is formulated by including the coupling between the vertical and the torsional motions. In the linearization procedure, the nonlinear terms are approximated as a sum of linear and constant terms in order to take into account the nonzero mean components as well as the fluctuating components of structural responses. The verification has been made on a case with four modal degrees of freedom. Example analyses have been carried out on two structures for various wind speeds and wind force parameters. Numerical results indicate that, by including the nonlinearity into the analysis, the dynamic responses of the bridges, particularly in the vertical direction, change considerably.