The main objective of this study is modeling the dependency structure between the returns of oil markets, exchange rate and stocks of chemical products in Iran. For this purpose, the theory of Vine Copula functions is used to investigate the dependency structure. In addition to consider a linear relationship between financial markets in Iran, the nonlinear dependency structure of these markets is also estimated, and their dependence on their upper or lower tails is determined. The study period includes daily data (5 working days) from December 2008 to July 2017. Modeling of marginal distributions of GJR-GARCH models has been used. Then, using the Copula-GARCH approach, the structure of dependency between returns and the calculating of the Value at Risk (VaR) of crude oil, exchange rate and stock of the chemical product group returns have been investigated. Finally, the required back-test is performed on the basis of the loss function. The study findings show that both pairs of modeling returns are related to the same upper and lower tails. In addition, there is a same structural dependency on the distribution of the vine copula between the indexes of chemical products and the nominal exchange rate on the condition of the price of crude oil, which indicates the spillover between markets. Due to that spillover effect is the main source of financial risk, the structural dependence on the basis of vine copula functions makes accurate and reliable calculation of portfolio risk based on the VaR criterion.