Statement of Problem: Different temperature and moisture around Iran and wide usage of stone type IV in dental laboratories in our country were the reasons for us for doing this study.

Purpose: In this survey the effect of temperature, moisture and time on dimensional change of dental stone type IV (Velmix) were investigated.

Materials and Methods: In this experimental study cube shape Metal models (10×10×10mm) was made for Taking impression. We used special tray, which has ten holes (15×20×25mm) and made impression with condensational silicone in two-step procedure, and poured it with stone type IV. stone cubes were divided to different groups. 3 groups holed in fix moisture and variable tempraure and 5 groups in fix temperature and variable moisture. After 2 hours, 24 hours and one-week dimension of stone cubes were measured. Then analysis was done with ANOVA and dauncan.

Results: The following conclusions were achieved: 1-Time has no effect on dimensional changes. 2-Tempreture and moisture has some effect on dimensional changes in stone IV. Increasing of temperature result in stone contraction and increasing in moisture result in stone expansion.

Conclusion: The best temperature for least dimensional change is 20C and the best moisture for a least dimensional change is 30 %.