The fracture toughness of directional solidified Ti-(45,47)Al-3Nb, Ti-(45,47)Al-3Nb-0.2Si-0.1C, Ti-(45,47)Al-3Nb-0.3Si-0.2C type I alloys and their contribution to crack growth resistance of TiAl-Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by alpha(2)-alpha(2) spacing in which the delamination-type separation occurred. Compared to 47Al alloys, alpha(2)-alpha(2) spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl-Nb alloys produced by directional solidification is suggested. (C) 2007 Elsevier Ltd. All rights reserved.