The nuclear reactor is designed following the concept of defence in depth (DiD). To ensure the adequacy of the concept is applied in the design, the fundamental safety function can be used as a reference. The fundamental safety function consists of three items: to control reactivity, remove heat from the reactor and confine the radioactive material. The design assessment can be done by determining the failure of the fundamental safety function using probabilistic analysis. PWR is a type of reactor that is mainly operated at this time. One new type of PWR is AP1000, which started commercial operation in 2018. Therefore, its safety level still needs to be assessed continuously. This research aims to evaluate the fundamental safety function by probabilistic analysis and assess the DiD concept capability in the AP1000. The analysis is done by making the fault tree of the fundamental safety function, whereas the assessment of defence in depth is carried out using matrix DiD level against the fundamental safety function. The first fundamental safety function, the second fundamental safety function, and the third fundamental safety function consist of six, five, and five intermediate events, which are analyzed till basic events. Furthermore, it is carried the failure combination of each level of fundamental safety function. The probability data of component failure or human error used in the analysis are collected from the international atomic energy agency (IAEA), Westinghouse, Nuclear Regulatory Commission (NUREG), and other published documents. The results show that the failure frequency for all fundamental safety functions is 4.36E-10 per year. The combination failure frequency of fundamental safety function for first and second, first and third also second and third, is 2.84 E-06, 2.31E-07, and 2.90E-07 per year. In contrast, the frequency of critical subsystems for fundamental safety functions is 5.18E-04, 9.11E-04, and 1.16E-04 per year. Based on the analysis, it was concluded that the failure frequency of the fundamental safety function for AP1000 is still within IAEA criteria, which are smaller than the frequency of infrequent faults and limiting faults classification. The AP1000 design followed the concept of defence in depth. [ABSTRACT FROM AUTHOR]