Chilling requirement, i.e. the minimum duration of the chilling treatment necessary for full vernalization, of a wheat cultivar should be evaluated by using a measurement which involves not only the growth after the treatment but also the growth during the treatment. Based on this concept, two assumptions were proposed in this study for the growth during the chilling treatment. Subsequently an index “Dof” and a plant-development model using the Dof (Fig.2) were developed on the basis of the assumptions. Through a series of experiments involving 15 wheat cultivars, the accuracy of the assumptions was verified, and the development model was found to be quite adequate for the evaluation of the chilling requirement and narrow-sense earliness. The fifteen wheat cultivars examined in the present experiments were classified into two groups, i. e. spring and winter wheat cultivars by the pattern of flag-leaf unfolding in the absence of chilling treatment. This classification revealed a distinct difference in the chilling requirements between the two groups, and enabled to conclude that the wheat cultivars which required maximum of 30 days of chilling and those which required 40 or more days can be designated as spring wheat cultivars and winter wheat cultivars, respectively. Among the spring wheat cultivars, three that had the Vrn 1 gene and two having the Vrn 3 gene required a chilling treatment of 0 and 30 days for full vernalization, respectively. This finding indicates that Vrn 1 makes wheat completely insensitive to the chilling treatment and that the chilling requirement controlled by Vrn 3 disappears by 30 days of chilling treatment. The experimental results showed that the chilling requirement precisely reflects the demand of a wheat culcivar for low temperature for reproductive growth. Therefore, it is suggested that the chilling requirement is the supremely important trait for understanding the nature and genetics of vernalization.