El Niño-Southern Oscillation (ENSO) seasonal phase-locking behaviors simulated in 36 Coupled Model Intercomparison Project Phase 6 (CMIP6) models are evaluated for the first time by comparison with 43 CMIP5 models and observations. There are much more aborted ENSO events (simulated mature phase occurring out of the winter season) in 30 CMIP6 and 33 CMIP5 models than in observations, which indicates that the reasonable ENSO seasonal phase-locking is still a challenge to state-of-the-art climate models. Furthermore, the seasonal cycle of the zonal SST gradient along the equator can explain approximately 30% and 36% of the variance in the ENSO phase locking for CMIP5 and CMIP6, respectively. Moreover, both the spatial distribution and the phase change timing of the zonal SST gradient seasonal cycle are crucial for the ENSO seasonal phase locking. Improvement of the simulating ENSO phase-locking should be realized by focusing on the seasonal cycle of the zonal SST gradient.