In the last decade, heavy fog events have been found to last from several days to more than a week and frequently occur in northern China. The mechanism that responsible for the long-lasting heavy fog events has not been fully understood. In this study, the radiative effects of the clouds and fog on two persistent heavy fog events on December 3–6, 2011 (Case-2011) and January 28–31, 2013 (Case-2013) are investigated based on both observations and Weather Research and forecasting (WRF) model. The results indicate that the radiative effects of clouds formed at middle and high levels and fog may play an important role in sustaining long-lasting fog events in northern China. The clouds formed above the fog primarily consist of ice and snow particles with a thickness of more than 3–4 km and cloud base of 3–6 km. During the daytime, the increase in solar radiation reflection caused by both clouds and fog can reduce the mean surface solar radiation by 71% in Case-2011 and 84% in Case-2013, and the contribution from clouds accounts for 18%, which may significantly weaken the surface heating from solar radiation and prevent the dissipation of fog events. While during the nighttime, the greenhouse effect of clouds and fog can increase the mean surface net radiation by 77 W m⁻² in Case-2011 and 68 W m⁻² in Case-2013, which may reduce the surface long-wave radiative cooling and prevent the further development of fog events. However, the greenhouse effect of clouds and fog during both daytime and nighttime may enhance turbulent processes and cause the fog to rise, and remain in the stable inversion layer