With the rapid development of electronic industry, thermal management has become a critical issue that severely restricts the application of portable devices. In this work, we fabricate a flexible and free-standing graphitized-graphene/polyimide I-g-GO/PI) film via an in-situ "molecular welding" strategy. With the help of in-situ polymerization, PI can be well-dispersed with GO and serves as a solder to enlarge the grain size of GO, resulting in an enhanced thermal conductivity of the film. The 7 wt % addition of PI into GO (I-g-GO/PI-7%) leads to an in-plane thermal conductivity as high as 1269.700 +/- 1.498 W/m/K, which is 81.8% higher than that of the pristine graphene and also superior to that fabricated via solution blending method by 58.3%. Simultaneously, the hybrid film exhibits an excellent flexibility and survives from a 2000 cycles bending test. The large-area hybrid film prepared by such an in-situ "molecular welding" method provides a promising way to fabricate graphene-based film for highly efficient thermal management. (C) 2018 Wiley Periodicals, Inc.