Alginate is the most widely materials used in dentistry. This impression material of alginate as a powder will become hydrosol if mixed with water and then become hydrogel arrowroot starch has been studied to be a mixture of alginate. The good impression material should has enough time for setting. The aim of this research is to know the effect of water/powder ratio towards the setting time of alginate impression material addition with arrowroot starch. The method of this research is experimental laboratories. There are 25 samples, that divided into 5 groups with different water/powder ratio (20.5ml, 19.5ml, 18.5ml, 17.5ml, 16.5ml). The data of analysis of this research using parametric One Way Anova. The result of this research show that the value of p<0.05 that means the water/powder ratio has effect towards the setting time of alginate impression material addition with arrowroot starch. The conclusion of this research is the lower of water/powder ratio, will make the setting time of alginate faster. The fastest of the setting time is water ratio at 18.5ml.