Glycemic index (GI) is used to classify carbohydrate-rich foods especially those containing at least 15% carbohydrates on the basis of their ability to raise blood sugar. Interest on the GI of carbohydrate-rich foods is rapidly growing due to the role it plays in the prevention and management of diabetes mellitus, a disease that is rapidly spreading worldwide. This study investigated the glycemic indices of cassava and sweet potato which are widely produced and consumed in Western Kenya as well as other parts of the country. The cassava and sweet potatoes were peeled, cleaned and boiled in water until tender. Proximate analysis of the samples was conducted according AOAC methodology and glycemic index was determined according to the methodology recommended by FAO/WHO using eight healthy volunteers. The results of the proximate analysis showed the carbohydrate content of cassava to be 90% and sweet potato at 78% on dry weight basis. Cassava had a GI of 74 while sweet potatoes had a GI of 65 although these GI values were not significantly different (p>0.05). Nonetheless both cassava and sweet potato had high glycemic load. Thus, they should be consumed in moderation by individuals suffering from diabetes mellitus.