The mechanism of coal-dust action on inhibiting burn-on of Sand to iron castings was taken in consideration by means of casting test and thermal decomposition test. To compare the ability of inhibiting sand burn-on, test castings were produced in green sand moulds added three different coal-rusts. And quantitative determination of lustrous carbon and volatiles production for coal-dust samples were performed. The lustrous carbon production was in good agreement with the casting test result. But total voltiles production was relatively inefficient on hibiting sand burn-on to test castings. The lustrous carbon theory can be given to explain the mechanism which coal-dust inhibits sand burn-on to iron castings.