ِAn experiment was applied in DRC Experimental Station at East Elqantara in North Sinai to test the effect of water application rate of 40, 60, 80, 100 and 120% of ETc and bentonite as soil amendment at rate of 0, 2, 3, and 4 ton/fed on growth, yield, chemical composition, water use efficiency and investment ratio of potato plants.  Result revealed that growth, tuber parameters and yield and its components were increased with increasing water and bentonite application. There were no significant differences between irrigation treatments of 100 and 120% of ETc or between 3 and 4 ton/fed bentonite in most studied parameters in both seasons. Also no significant effect was found on chemical components, except P, which was significantly increased with 100% of ETc in the second season. Moreover, the maximum WUE values were contributed to high bentonite treatments i.e., 3 and 4 ton/fed combined with either 80 or 100% of Etc. Significant positive correlations were found between total yield and either average tuber weight or number of tubers/plant. Also, between tuber number/plant and number of aerial stems/plant. Results of investment ratio and multiple regression of total yield on both amount of irrigation water and bentonite rate led to conclude that there were two options to get sufficient investment ratio.