Silicon coatings produced by atmospheric plasma spraying are used as bond coat in environmental barrier coatings which protect silicon carbide fiber reinforced silicon carbide composites against water vapor induced corrosion at high temperatures. Silicon coatings exhibit structural defects such as interlamellar boundaries, gaps, pores and microcracks, some of which become pathways for oxygen penetration, leading to internal oxidation of the silicon coating. This study was conducted to reduce the pathways for oxygen penetration by heat-treatment under argon atmosphere and investigate the effects of heat-treatment on the structure, fracture toughness and oxidation behavior of the silicon coating. The results show that the structural defects could be reduced by a heat-treatment at T = 1300 °C, which resulted in a lower mass gain of the coating in the initial cycle in a cyclic oxidation test. The fracture toughness of the coating was significantly decreased by the heat-treatment, which led to the formation of microcracks in the coating in the cyclic oxidation test. As a result, the heat-treated coating exhibited a higher mass gain rate than the as-sprayed coating, leading to that both coatings exhibited almost the same mass gain at the end of the cyclic oxidation test of n = 10 cycles.