To determine the thickness of coronal hard structure the minimal distance between pupal surface and outer surface of crown was measured by means of Bowley gauge on extracted first molars. Upper(28 teeth) and lower(24 teeth) were carefully collected from 30-39 years of age and male. The teeth were split mesio-distally through central pit. On the split surface various part which are deeply related in cavity preparation were measured (schematic drawing). The results were as follows: A : Distance from mesio-cervical enamel to pulp chamber surface. upper $2.63{pm}0.19$(mm) Lower $2.18{pm}0.27$(um) B : Distance from mesial chamber ceiling to mesial surface upper $2.75{pm}0.34$ Lower $2.62{pm}0.31$ C : Distance from mesial chamber ceiling to occlusal surface upper $3.82{pm}0.51$ Lower $3.49{pm}0.50$ D : Distance from distal chamber ceiling to occlusal surface upper $4.28{pm}0.69$ Lower $3.90{pm}0.52$ E : Distance from distal chamber ceiling to distal surface upper $2.79{pm}0.45$ Lower $2.41{pm}0.40$ F : Distance from disto cervical enamel to pulp chamber surface upper $2.49{pm}0.24$ Lower $2.39{pm}0.25$.