The PnetCDF project started in 2001, independently from the Unidata’s NetCDF project. Applications can use PnetCDF library completely without NetCDF library. The initial goal of PnetCDF is to develop a parallel I/O library for applications to access CDF-1 and 2 formats on parallel computers. Its focus is to achieve high I/O performance. The design adopts a new set of APIs (with prefix name of “ncmpi_”) due to its implementation being tightly coupled with MPI. To encourage adoption from NetCDF users, the syntax of PnetCDF APIs stays mostly the same as the NetCDF’s. Through fully use of existing optimizations available in MPI-IO implementation, PnetCDF has been demonstrated to be able to deliver high-performance parallel I/O.