Background – Rupiah is one of the weakest currencies in the world. Theoretically, the domestic currency depreciation will increase exports and attract capital inflows. However, over the past few years, the trade balance is negative and economic growth is stagnant at 5 percents. Given that Indonesia is a small and open economy, the Rupiah fragility would have significant effect towards monetary stability as pre-requisite to economic growth. Therefore, survelliance assessment in Rupiah fluctuation is important by building a credible index as surveillance tool. Problem Statement – This paper aims to build a composite index, called Rupiah Fragility Index (RFI), as surveillance tool for detecting currency risk. Hence, the paper addresses the issue on how is to build the Rupiah Fragility Index in Indonesia? Literature – The current literature rarely measures the optimal level of rupiah fluctuations. The study suggests the optimal range for Rupiah in order to finally prevent the future currency crises which may disrupt the economy at large. Methodology – The study adopts two indicators as developed by Kaminsky (1998) where he defines a crises as the condition of currency depreciation and foreign reserve depletion. An extraction signal approach is utilized with various time horizons and threshold to find out the oiptimal level of fluctuations. Expected Finding – The RFI shows that Rupiah di fluctuationg over the observation periods since 1990 until 2019 in monthly basis. The extraction signal approach is capable to suggest the optimal level of fluctuation. Conclusion – The index is important as surveillance tools for policy intervention.