We present the formation of dislocations in thick Ge quantum dot superlattices and the effect of these dislocations on Ge quantum dots. Photo luminescence, TEM and AFM were used to characterize the Ge quantum dot superlaittices. For Ge (1.5 nm)/Si (20 nm) superlattices, it was found that the critical thickness was reached when the film thickness was around 0.5 mum. The threading dislocations were formed in the film afterwards and induced circular holes on the surface. Ge quantum dots tended to nucleate at the edge of the holes. The limited critical thickness for Ge quantum dot superlattices is a limitation for applications, such as optoelectronics and thermoelectronics. (C) 2003 Elsevier Science B.V. All rights reserved.