Ombus-ombus is one of the traditional Batak Toba foods from North Tapanuli made from glutinous rice flour. The nutritional content of purple sweet potato is quite complete at affordable prices, which has the potential to be developed into processed food products based on local food and can reduce imports of Indonesian glutinous rice. The main objective of this research was to determine the best ombus-ombus cake formulation by using purple sweet potato as a partial substitute for glutinous rice flour. This research was conducted using a completely randomized design, consisted of two factors, factor I, the percentage substitute of purple sweet potato flour (P1 = 0%; P2 = 10%; P3 = 20%; P4 = 30%) and storage temperature condition (S1= at room temperature and S2= at freezing temperature) with three replications. The moisture content of ombus-ombus based on research was very significantly different (P <0.05) where the value was higher ranged 40-45%, and the organoleptic of colour, aroma, taste, texture were also very significantly different, almost the same value as previous research values and preferred by panellist than commercial products. The best formulation is 20% purple sweet potato flour and 80% glutinous rice flour with freezing storage.