An analysis is carried out to investigate the effects of Joule heating and thermal diffusion effects on an unsteady MHD heat and mass transfer free convection Casson fluid flow past an oscillating semi-infinite vertical moving porous plate in the presence of heat source/sink and an applied transverse magnetic field. The governing flow is modeled in the form of partial differential equations with initial and boundary conditions. With suitable non-dimensional quantities the governing non-linear partial differential equations obtained in dimensionless form, which are solved numerically with finite difference scheme. Numerical results for non-dimensional velocity, temperature and concentration as well as the skin-friciton, the rate of heat transfer and the rate of mass transfer studied for different physical parameters. The results show that that the solutal boundary layer thickness of the fluid enhances with the increase of Prandtl number and the temperature is increased by an increase in the heat source by the fluid. The central reason behind this effect is that the heat source causes an increase in the kinetic energy as well as thermal energy of the fluid. The momentum and thermal boundary layers get thinner in case of heat source fluids.