Modelling of extra large flat jack (ELFJ) testing at the Bakhtiyari dam site, Iran is the subject of this study. A complete 3D model of dam abutment, exploration gallery, and test set up was constructed. The aim was to reproduce the in-situ test results and determine the extra large jack coefficients manufactured by INTERFELS Corporation. In typical practice the large flat jack (LFJ) coefficients proposed by LNEC is used to calculate the final modulus data. These coefficients are also used to calculate the modulus data from ELFJ, which leads to significant errors. In this study a precise 3D numerical model of typical flat jack test condition was constructed using the Flac-3D code. Accordingly, the flat jack tests conducted at Bakhtiyari dam site, Iran, was simulated numerically. A very good agreement was achieved between numerical and field test results. Moreover, the obtained numerical results provided the coefficients of extra large flat jack. These, coefficients were employed for calculating the in-situ deformation modulus of Bakhtiyari dam rock mass.