We are developing an operating system, CEFOS in order to realize an efficient parallel and distributed computing environment based on a fine-grain multi-threading model. In CEFOS, a user process consists of a set of threads which are executed concurrently according to thread dependence. A thread in CEFOS is very fine-grain compared to a conventional thread such as Pthreads. By consisting a process with fine-grain threads, we can employ flexible scheduling schemes as long as thread dependence allows. In this paper, we propose Wrapped System Call mechanism using a cooperation mechanism between user-mode and kernel-mode. This mechanism aggregates multiple system-calls and executes them as a single system-call. In order to reduce overhead of context switches and mode changes between user-mode and kernel-mode. We also show the performance evaluation results and discuss the effectiveness of our mechanism.