There are several factors that cause project delays it can from the owners, contractors, consultants or external factors. Project delays not only occur in public and private projects but also in projects whose funding sources come from foreign loans and grants. One of the projects funded by foreign loans in Indonesia is the National Slum Upgrading Program (NSUP) in Banten, Indonesia. The NSUP is funded through a foreign loan from the Islamic Development Bank (IsDB). There are three area-scale NSUP activities in that experienced construction delays. The objective of this paper is to determine the factors causing construction project delays. The research was conducted by distributing questionnaires to 11 respondents of project owners, contractors and consultants of thr NSUP project. Methodology of data analysis with validity, reliability tests and ranking factors through the Relative Importance Index (RII) analysis. The results showed that there are ten dominant project delay factors including the Covid-19 factors, and also five dominant work items that cause project delays. In conclusion, there must be watched out for and anticipated by all project parties so that project delays can be minimized and even avoided. This anticipation can be not only from the planning phase, design, auction, and finally to the implementation of the project, but also all possible force majeures such as pandemic.