Conventional software reliability models are not adequate to assess the reliability of software system in which OSS (Open Source Software) adopted as a new feature add-on because OSS can be modified while the inside of COTS(Commercial Off-The-Shelf) products cannot be changed. This paper presents an approach to software reliability assessment of OSS adopted software system in the early stage. We identified the software factors that affect the reliability of software system when a large software system adopts OSS and assess software reliability using those factors. They are code modularity and code maintainability in software modules related with system requirements. We used them to calculate the initial fault rate with weight index (correlated value between requirement and module) which represents the degree of code modification. We apply the proposed initial fault rate to reliability model to assess software reliability in the early stage of a software life cycle. Early software reliability assessment in OSS adoption helps to make an effective development and testing strategies for improving the reliability of the whole system.