Abstract: In order to understand the genetic control of nicotine conversion in burley tobacco the joint genetic analysis of nicotine conversion was carried out based on four generations P1 (Va1050 with high nicotine conversion), P2 (Va1053 with low nicotine conversion) and their F1 and F2 using mixed model of major gene and polygene. Genetic parameters were also estimated. The results showed that the genetic model of two pairs of major genes with equal additive and equal dominant effect and polygenes with additive and dominant effect (E-5) was selected for nicotine conversion of burley tobacco materials used in this study. The heritability of major genes and polygenes was 58.18% and 11.72%, respectively. As a consequence, nicotine conversion of burley tobacco mainly was controlled by two pairs of major genes and modified by polygenes and environmental factors such as temperature and humidity. In practical breeding, it will be effective that selecting parents with low conversion for breeding target varieties because of high heritability of this trait. Additionally, temperature and humidity control are also very important for reducing nicotine conversion during tobacco leaves curing and aging.