We determined the effects of carbon concentration on the microstructures and tensile properties of the Ni-free Co-29Cr-9W-1Si-C (mass%) cast alloys used in dental applications. Alloy specimens prepared with carbon concentrations in the range 0.01-0.27 mass% were conventionally cast. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) revealed that precipitates had formed in all the alloy specimens. The σ phase, a chromium-rich intermetallic compound, had formed in the region between the dendrite arms of the low-carbon-content (e.g., 0.01C) alloys. Adding carbon to the alloys increased the amount of interdendritic precipitates that formed and changed the precipitation behavior; the precipitated phase changed from the σ phase to the M23C6 carbide with increasing carbon concentration. Adding a small amount of carbon (i.e., 0.04 mass%) to the alloys dramatically enhanced the 0.2% proof stress, which subsequently gradually increased with increasing content of carbon in the alloys. Elongation-to-failure, on the other hand, increased with increasing carbon content and showed a maximum at carbon concentrations of ~ 0.1 mass%. The M23C6 carbide formed at the interdendritic region may govern the tensile properties of the as-cast Co-Cr-W alloys similar to how it governed those of the hot-rolled alloys prepared in our previous study.