The growth characteristics of thermally grown oxides (TGO) and their influence on microcracking in an air-plasma sprayed thermal barrier coating (TBC) were studied. The TBC samples were prepared in either as-received condition or with a pre-heat treatment. In the as-received TBC, TGO that was formed upon thermal exposure predominantly consisted of layered and clustered chromia, spinels and nickel oxide, whereas in pre-heat treated samples the TGO was predominantly alumina. The growth characteristics of TGO was found to exhibit a three-stage behavior that was most pronounced in the as-received TBC. Micro-cracks were found to nucleate in clustered oxides, these cracks would grow in association with thickening of the TGO layer. Eventually, oxide-induced cracking and cracking along pre-existing discontinuities near the ceramic/bond coat interface led to spallation of the topcoat. A relationship between the maximum crack size and TGO thickness was established, based on fracture mechanics considerations. This relationship is shown to be useful for TBC life prediction.