In this work, two isothermal sections of the Co-Ta-Si ternary system at 900 °C and 1100 °C are constructed in the whole composition range via phase equilibrium determination with the help of electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques. Firstly, several reported ternary phases G (Co₁₆Ta₆Si₇), G″ (Co₄TaSi₃), E (CoTaSi), L (Co₃Ta₂Si) and V (Co₄Ta₄Si₇) are all re-confirmed again. The G″ phase is found to be a kind of high-temperature compound, which is unstable at less than 1100 °C. Additionally, the L phase with a large composition range (Co_32–62Ta_26–36Si_10–30) crystallizes with a hexagonal crystal structure (space group: P6₃/mmc, C14), which is the same as that of the binary high-temperature λ₁-Co₂Ta phase. It can be reasonably speculated that the ternary L phase results from the stabilization toward low-temperature of the binary λ₁-Co₂Ta through adding Si. Secondly, the binary CoTa₂ and SiTa₂ phases are found to form a continuous solid solution phase (Co, Si)Ta₂ with a body-centered tetragonal structure. Thirdly, the elemental Si shows a large solid solubility for Co-Ta binary compounds while the Ta and Co are hardly dissolved in Co-Si and Ta-Si binary phases, respectively.