Polycarbonate (PC) was blended with various loadings of novel phosphorus-containing thermotropic liquid crystalline copolyester named PHDDT to form the in situ reinforced composites (PC/PHDDT) by flake extrusion. The morphology, thermal behaviors, tensile properties and flame-retardant performances of PC/PHDDT composites were investigated. Results suggested that fine deformation and microfibrillation of PHDDT in PC matrix could be formed during flake extrusion, which was confirmed by both the SEM observation and rheological analysis. With the increase of PHDDT content, the limiting oxygen index (LOI), tensile strength and storage modulus of the composites were enhanced simultaneously, along with the gradually decreased values of the peak heat release rate (PHRR) and the total heat released (THR), indicating the in situ reinforced and flame-retardant PC/PHDDT composites could be obtained. (C) 2011 Elsevier Ltd. All rights reserved.