In this paper, the concept of directional smoothing is introduced and its application to interferogram noise reduction is presented. Interferograms provide isophase lines, that is, fringes and they thus contain directional information. In essence, the method incorporates this valuable directional information of interferograms by setting up a slender mask of relatively large aspect ratio along a fringe. The new value, that is, the average or median intensity of the pixels within the slender mask is assigned to each pixel. The slender mask could be straight or curved. For a straight slender mask, one should find the average direction of fringes within a certain chosen region. For a curved slender mask, the mask is set up along the fringe direction and may vary for each pixel. Based on computer simulation of experiments, the results appear to be promising as compared with ordinary smoothing or median filtering.