New TiAl alloys, containing 45 at.% Al, 7 at.% Nb, x at.% W, and 0.15 at.% B (x = 0, 0.2, 0.4, and 0.7) were prepared by arc melting and drop casting consequently. Using optical microscopy, scanning electron microscopy (SEM), and electron superprobe technologies, the effects of tungsten on the microstructural evolution of the TiAl alloys, including the colony size and lamellar spacing, were analyzed. It was found that cellular structures and dendrites were formed in the as-cast TiAl alloys, and heavy metals, such as niobium and tungsten, tend to segregate strongly at the interface of the cellular structures and dendrites. Trace tungsten can effectively impede the grain growth and narrow the interlamellar spacing. 0.4 at.% tungsten is more effective in refining the microstructure of the TiAl alloys.