The flexible electrothermal heaters based on the Joule heating effect have attracted extensive attention in recent years due to their wide range of applications in personal thermal management and wearable devices. However, the fabrication of this kind of flexible heater with high performance but at low cost remains a challenge. Here in this work, a highperformance and flexible electrothermal heater was developed through layer-by-layer self-assembly of graphite nanoplates (GNPs) and carbon black (CB) on paper driven by capillarity. The resistivity of the assembled GNP/CB layer was around 0.02 Ω•cm after dip-coating 10 times. The heater reached a maximum temperature of 197.3 °C with a heating rate of 23 °C s −1 under a relatively low voltage of 6 V. Importantly, no attenuation of the heating performance was observed under deformation of the device. This flexible electrothermal heater may find versatile applications, including water heating, deicing, wearable devices, and medical treatments.