We study the use of the wavelet expansion in terms of Meyer's wavelet to the problem of mollification in the numerical calculation of fractional derivative. It is shown that, when the simplest of Meyer's wavelet is used, the expansion is equivalent to the transform by the de la Vallée Poussin kernel, that was proposed by Hào et al. It is expected that better results are attained by using the rapidly decaying harmonic wavelet, which is another of Meyer's wavelet. We examine this. It is also shown that there exists an estimation of the error of approximation in the mollification by using only simple inequalities, without sophisticated inequalities which are invoked in the paper by Hào et al. Examples of numerical calculation are given.MSC 2010 : 26A33, 65D25, 65J20, 42B35