A Monte-Carlo simulation method is proposed which can take uncertainties of scale and location parameters of Gumbel distribution into account straightforwardly in evaluating significant design wave heights with respect to return periods. The uncertainties of design wave heights may directly depend on the amounts of uncertainties of scale parameter and those distributions may be followed by Gumbel distribution. In case of that the expected values of maximum significant wave height during lifetime of structures are considered to be the design wave heights, more uncertainties are happened than in those evaluated according to return periods with encounter probability concepts. In addition, reliability analyses on the armor units are carried out to investigate into the effects of the uncertainties of design wave heights on the probability of failure. The failure probabilities of armor units to 5% damage level for 50 return periods are evaluated and compared according to the methods of taking uncertainties of design wave heights into account. It is found that the probabilities of failure may be distributed into wide ranges of bounds when the uncertainties of design wave heights are assumed to be same as those of annual maximum significant wave heights.