The pre-mixing Diesel-CNG was suggested to increase the air quantity inside the cylinder more than if a mixture of CNG-air is injected in the engine which may result is a rich mixture, directly. The objective of the present paper is to study the behaviour of gas bubbles in the liquid fuels and the ways to control the bubbles sizes prior to injection by an external magnetic field. Numerical simulation of Diesel-CNG bubbly flow with effecting magnetic field is presented in this paper. The three-dimensional incompressible Navier-Stokes equations have been used to solve the CNG-Diesel two phase flows in 20 mm ID horizontal pipe. The simulation was carried out using ANSYS fluent software and the flow field discretization was achieved by Volume-of-fluid method (VOF) technique. The numerical results showed that CNG bubbles tend to move toward the upper wall under the buoyancy force effect and these bubbles grow to a bigger size and expand vertically in the Diesel flow before it breaks away with effecting magnetic field 0.4 to 0.8 Tesla, respectively, and the gas volume fraction values increased with noted to by increasing the magnetic intensity. The laminar behaviour of the flow changed in upper zone from the pipe to increasing gas volume fraction, while the axial Diesel velocity decreases and the profiles tend to flatten with increasing of the magnetic field strength. The numerical results have been validated by comparing the computational results with experimental results reported in the literature where a good agreement was achieved. ©2006-2016 Asian Research Publishing Network (ARPN).