The flowering dynamics of Ocimum basilicum L., (a common population maintained at the UHFI) was studied with the aim to create an exact and practically applicable method for definition of phenological phases. According to our observation the development of individual flowers of the basil can be characterized by 8 distinguishable phases, which must be considered for description of the actual phenological stage of a spike. An accurate model was created for the unambiguous description of flowering process of different flowers within the spike and for the individual plant as a whole. The new flowering index formula is calculated from the number of flowers weighed by their phenological phases. The time dependence of flowering is presented by functions fitted to values of the flowering index. The results reflected different patterns of the main inflorescence, the inflorescences formed on the side shoots in the first or in the second half of the flowering period. However, for description of the flowering process of the whole plant, a sigmoid function proved to be the appropriate model. The accumulation process of the essential oil could be characterized by the flowering index values. They showed close correlation (r=0.964) at high probability. The new method assures an exact definition of phenological phases in basil. Its application seems to be optimal during breeding procedures, seed production and even in production of high quality drugs, according to the requirements of the Good Agricultural Practice (Guidelines for GAP, 1998).