This talk presents an overview of the Petascale System Interconnect (PSI) project. The PSI project is one of the national projects on "Fundamental Technologies for the Next Generation Supercomputing" of MEXT (Ministry of Education, Culture Sports, Science and Technology), Japan. The goal of the PSI project is to develop technologies enabling petascale supercomputing systems with hundreds of thousands of computing nodes. The PSI project consists of three subprojects to tackle with the three fundamental technologies: subproject 1 is for the small and efficient optical packet switches; subproject 2 is for the low-cost & high-performance MPI communications; and subproject 3 is for the methodologies of evaluating and estimating the performance of petascale systems. With the successful completion of the PSI project, the Japan Next-Generation Supercomputer R&D Center (NSC) will take the technologies to build Japan's next generation supercomputer, which is expected to be over 70 times faster than the current fastest supercomputers.