Due to depletion of crude oil and exhaust emissions associated with internal combustion engine, biodiesel, neat vegetable oil and waste cooking oil are identified as potential alternative fuels to run on diesel engines. However, the viscosities of these fuels are higher than diesel and can be grouped as higher viscous fuel (HVF). Currently, diesel engines fuelled by HVF experience problems of reduced power and torque besides increased fuel consumption and in-cylinder carbon deposit. These are mainly due to poor combustion as HVF is less prone to evaporate and mix with air. To reduce these problems, a technique to improve the air-fuel mixing in diesel engine fuelled by HVF using Guide Vane Swirl and Tumble Device (GVSTD) is presented in this paper. Validated simulation model for a diesel engine was developed using Solidworks and ANSYS-CFX before 12 GVSTD models were imposed in front of the intake runner with the vane twist angle varied from 3° to 60°. Based on simulation results, the maximum improvements were found by the GVSTD of 35° twist angle. These improvements were about 0.02% in-cylinder pressure, 2.7% turbulence kinetic energy and 1.7% in-cylinder velocities than the base model without GVSTD. These parameters are expected to reduce the penetration length and increase the cone angle of the higher viscous injected fuel with resulting improvement of air-fuel mixing and reduce formation of carbon deposits on the surface of the combustion chamber.