A triclinic phase of tricalcium silicate (TI) was investigated using transmission electron microscopy (TEM), Electron diffraction patterns were analyzed by introducing a subcell with the cell parameters of a = 7.081 Angstrom, b = 7.043 Angstrom, c = 25.230 Angstrom, alpha = 89.97 degrees, beta = 90.37 degrees, and gamma = 119.44 degrees, It was proven that the coordinates of all the reflections can be expressed to be ha* + kh* + lc* +/- m/6(a* + 2b* + 2c*), where m = 0, 1, 2, and 3, The result indicates that the structure modulation in TI is a one-dimensional type with a structural modulation normal to (122), The modulated structure could be observed in a high-resolution TEM image as wavy contrast streaking parallel to the plane with an interval of six times the (122) spacing.