Thermal stability of TiA1-1.5W-Si and TiAl-1.5Mo-B alloys was investigated by partial melting in floating zone (FZ) furnace. The lamellar stability of TiAl-1.5Mo alloys, which proved to be thermally unstable, was improved dramatically by Si addition. Lamellar stability can be improved by preventing double right arrow-phase formation from double left arrow-phase due to the effect of shifting phase diagram to the low Al side by Si addition. However, it was found that B addition in TiAl-1.5Mo alloys had no affirmative effect on lamellar stability Contrary to Si addition, phase diagram cannot be shifted by B addition. Therefore, double right arrow-phase formation from double left arrow-phase cannot be prevented by B addition. In the result of partial melting, it can be concluded that in order to have enough lamellar stability as a seed alloy, the elements that can shift phase diagram to low Al side must be added to Ti-Al-Mo alloy. In addition, lamellar orientation control was successfully performed by directional solidification using the Ti-46Al-1.5Mo-1Si alloy as a seed alloy.