[en] A fully ceramic micro-encapsulated(FCM) fuel based on the dispersed particle fuel concept was considered on the one of accident tolerant fuel(ATF). A mixed core is established using the FCM fuel in the existing LWR core where UO2 fuel pellet was loaded. In order to demonstrate the thermal-hydraulic compatibility of the FCM fuel in the existing LWR core, pin by pin analysis is performed for transition period from mixed core with FCM fuel and UO2 fuel to the FCM fuel only. Parallelized MATRA code using MPI is developed for pin by pin calculation. Pin by pin analysis on mixed transition core for FCM fuel and reference UO2 fuel was performed on a quarter core with 13310 subchannels in assistance with the parallel algorithm with MPI with 20 cores. The thermal margin for the pin by pin model was evaluated by employing a quarter-core power distribution data provided by MASTER code that is nodal code. The pin by pin model shows the feasibility of mixed transition core of FCM fuel assembly based on the MDNBR results