A series of combined impressions, alginate impressions and rubber base impression made of the lower left quadrant of one subject having a cast gold crown with six reference marks on lower let 1st molar. the combined impressions of hydrocolloid were removed from the mouth at 2.5min, after the insertion of the tray. The alginate impressions were removed from the mouth at 2min, after the insertion of the tray. The rubber base impressions were removed from the mouth at 10min. The effect on the accuracy of the combined impressions, alginate impressions and rubber base impressions was obtained following results by comparing the distances between the reference marks in indirect stone cast with the standard measurements of the cast gold crown prior to cementation. The findings through this study could be summarized as follows; 1. The accuracy of combined impression was superior to that of alginate impression, but it was inferior to that of rubber base impressions. 2. Combined impressions produced a little distortion of the mesio-distal length, alginate impressions produced the largest distortion, ut rubber base impressions produced the least distortion.