The topologically close-packed (TCP) precipitation in the secondary reaction zone (SRZ) in a thermal barrier coated alloy CMSX-4 has been investigated. The selected area electron diffraction (SAED) and high-angle annular dark field (HAADF) were used to analyze the TCP precipitates in the three different regions (including the SRZ, grain boundaries, and substrate) in this study. It was found that the interface between the SRZ and substrate was a high-angle grain boundary with a misorientation of similar to 51 degrees. The TCP precipitates at the grain boundaries could inhibit the growth of SRZ. The SAED results indicate that most TCP precipitates were the faulted or twinned mu phase. In addition, the intergrowth of P phase with mu phase was found.