In order to study the phenology of weeds in the wheat fields and determining suitable indices for predicting phenology events and using the results for improvement chemical control, a trial was conducted in 2006-2007 growting season at Zanjan, Karaj, Varamin and Mahmoud Abad (Mazandaran). The seeds of weeds (Broadleaved and grasses) were gathered in previous season. For satisfied emergence, the weed seeds were treated for breaking dormancy 24 hr before planting. Phenological events of wheat and weeds were recorded during growth season. Thermal time based on growing degree days (GDD) was calculated for each growth stage. Phenological and thermal time results of wheat and weeds compare with the optimum time application of recommended herbicide for weed control of weeds. Integrated results showed that GDD was a suitable index for determining and predicting of wheat and weeds growth stages. Also GDD could use for precision time of application of herbicides and could recommend that 400 GDD and 1200 GDD were minimum and maximum of herbicide application in wheat fields, respectively. Irrigation of field before planting and tillage applicable for control of the weed seeds without dormancy.