Metal nanowires have been considered as essential components for flexible transparent conducting electrodes (TCEs) with high transparency and low sheet resistance. However, large surface roughness and high interwire junction resistance limit the practical use of metal wires as TCEs. Here, we report Cu microbelt network (Cu MEN) with coalescence junction and low surface roughness for next-generation flexible TCEs. In particular, the unique embedded structure of Cu MBN in colorless polyimide (cPI) film was achieved to reduce the surface roughness as well as enhance mechanical stability. The TCEs using junction-free Cu MBN embedded in cPI exhibited excellent mechanical stability up to 100 000 bending cycles, high transparency of 95.18%, and a low sheet resistance of 6.25 si sq(-1). Highly robust Cu MBN-embedded cPI-based TCE showed outstanding flexible heater performance, i.e., high saturation temperature (120 degrees C) at very low voltage (2.3 V), owing to the high thermal stability of cPI and excellent thermal conductivity of the Cu MBN.