Abstract: In the present work,the in-situ synthesized ultrafine WC-Co composite powder was presintered at 1 250,1 300,1 350,and 1 380 ℃,respectively. Then,the pre-sintered solids were sintered by the hot isostatic pressing(sinter-HIP) at 1 450 ℃ for 30 min in the Ar atmosphere with gas pressure of5 MPa. Microstructures and mechanical properties of the pre-sintered solid and finally sintered bulks were investigated,respectively.Resultsshow that the pre-sintering process can effectively control the abnormal grain growth. After pre-sintering treatment,the bulk materials prepared by sinter-HIP have excellent fracture toughness,with the maximum value of 16. 1 MPa·m^-1/2. The composite powder pre-sintered at 1350 ℃ and 1 380 ℃,sintered by HIP,has more homogeneous microstructure and high transverse rupture strength.