Effects of silicon in titanium carbide (TiC) have been investigated using high-resolution electron microscopy combined with high-spatial-resolution analytical electron microscopy. The results demonstrate that Si can reduce the twin boundary energy of TiC, leading to the formation of a lot of two-dimensional (2D) defects in TiC grains containing Si. These defects were identified as microtwins of four (111) spacings thick and structural-related Ti3SiC2 platelets of only one unit cell thick. The twin-stabilizing effect of Si is discussed in terms of coordination environments of Si. Very thin Ti3SiC2 platelets are formed, accompanied by the segregation of Si atoms and carbon vacancies to the twin boundaries. Microtwins and Ti3SiC2 platelets were found to grow in 2D with Si totally confined in the defects.