While both the timing and altitude of the diurnal cloud cycle influence Earth's energy balance and play a critical role in our climate system, there is insufficient literature on the diurnal characteristics of cloud cover at various altitudes. Here we compared total cloud cover modeled by the Coupled Model Intercomparison Project Phase 6 (CMIP6) and from International Satellite Cloud Climatology Project (ISCCP) simulator data, focusing on their diurnal variations at different altitudes through the Cloud Feedback Model Intercomparison Project (CFMIP). Results show significant discrepancies in the absolute values of total cloud cover between the models and the ISCCP simulator data. Time series analysis further suggests high correlations in total cloud cover between the model outputs and the ISCCP simulator data, especially in low-latitude regions. The results imply that less accurate variations of absolute cloud amounts but more reliable trends of total cloud cover change over time from model outputs. Significant diurnal cycles in low and high clouds were also uncovered, which are believed to have contrasting cooling and warming effects. These findings underscore the importance of modeling diurnal cloud cycle at different altitudes for improving climate predictions and weather forecasts.