Dielectrophoresis (DEP) based microdevices offer a great number of significant advantages for the manipulation of biological particles such as cells, bacteria, viruses and DNA over traditional methods. To enable successfully dielectrophoretic manipulation of biological particles, electric fields of higher intensity need to be generated in order to increase the DEP forces. However, the introduced electric field may cause the joule heating effect and thermal denaturation of biological particles. This paper presents a numerical solution of the DEP force and the resulting electrical electrothermal driven fluid flow on a DEP microdevice. Theoretical investigations were made about the impact of electrothermal flow on DEP based microdevice. The fluid field was solved by coupling electrical, thermal, and mechanical equations. It is shown that under some typical experimental conditions of DEP based manipulation of biological particles, it is necessary to consider the possible influence of the electrothermal flow.