Abstract: Based on the data of Guangfo Expressway, the different average speeds and densities taken separately from different flow sections, choosing 15min, 5min and 1min as statistical intervals, are compared one another and so are the maximum frequency speeds and densities from the same flow sections. Then Greenshields model and Under wood model are calibrated on the basis of statishcal results, also choosing 15min, 5min and 1min as the statistical intervals. Finally the determined conclusion is made:5min is the minimum statistical interval for calibrating traffic models, while for the traffic models precision prescribed less than 10%, 1min statistical interval is workable.