Abstract: Weather conditions affect driving safety.Study on the characteristics of the driver workload in bad weather conditions,which can improve the driving safety.A random sample of 3 drivers was tested in a simulator cabin that simulated driving on a freeway under four scenery of weather conditions which are clear day,moderate rain,moderate rain-fog (Visibility 100 m) and moderate rain-fog (Visibility 50 m).Index of heart rate variability (HRV) is adapted to measure driver workload.We found the driver workload varies in different weather conditions and it is higher with the worse of the bad weather condition.Visibility of the driving environment has the most influence on the driver workload.