The present study was conducted to investigate the effect of overfilling and burnishing upon the structure of occlusal margins of lathe cut and spherical alloy amalgam. All the experimental fillings were made in cavities (5 mm in diameter, 3.5 mm in depth and 120° in cavosurface angle) cut in cylinders of plexiglass. The condensation was completed in following three different techniques. 1) The cavities not overfilled, the margins not burnished. 2) The cavities overfilled, the margins not burnished. 3) The cavities overfilled, the margins burnished. The amalgam fillings in the cavities were embedded, sectioned and polished. Microphotographs of the sections of the filling margins were enlarged (328 times) and copied on paper. The porosity was determined on the enlarged prints using the line-sectioning method. In the group of lathe cut alloy amalgam, the differences of the percentage porosity between marginal technique 1 and 2, or 2 and 3 were statistically significant, while in the group of spherical alloy amalgam, there were no significant differences between technique 1 and 2 or 2 and 3. Comparing the two groups, lathe cut and spherical alloy amalgams, technique 1 showed highly significant difference and technique 3 showed probably significant difference but technique 2 showed no significant difference. According to the results of this study, overfilling and burnishing were significantly decreased the porosity of occlusal margins of lathe cut alloy amalgam filling. In the case of spherical alloy amalgam, overfilling and burnishing were not so effective to decrease the porosity.