Accident tolerant fuel (ATF) materials need to be studied by subchannel analysis under a variety of transient conditions before being applied to Light-Water-Reactors (LWRs). The present study assesses the thermal behaviors of various ATF claddings and fuels under four different transient conditions which will give rise to the occurrence of Departure from Nucleate Boiling (DNB) phenomenon. A 5 × 5 rod bundle model is built to conduct the subchannel analysis, and maximum cladding temperature (MCT) and maximum fuel centerline temperature (MFCT) of different ATF cladding and fuel groups are obtained. The simulation results show that both MCT and MFCT are reduced with the utilization of FeCrAl cladding, superior to HNLS/ML-A and SA3/PyC150-A claddings that only reduce MCT but increase MFCT. Significant MFCT reduction but slight MCT increase are achieved in most transient cases with the application of UO2 + BeO/SiC fuel pellets. Due to the different safety limit of various ATF materials, safety margin analysis is performed as well. The results show FeCrAl possibly is the best alternative cladding material among claddings studied, because both cladding safety margin (CSM) and fuel pellet safety margin (FPSM) are improved. Compared to single UO2 fuel, a larger fuel pellet safety margin is still provided by UO2 + BeO/SiC fuel pellets.