Abstract Atomic force microscopy imaging of a supported bilayer of the phospholipid DPPC revealed the presence of thin lines, which were thought to be the boundaries of domains with a different orientation. Temperature-controlled AFM showed that the melting from the gel state Lβ′ to the fluid Lα phase starts on these lines. The observed onset of melting at 40.3 °C compared well to reported DSC measurements. The same mechanism of melting was observed in computer simulations on a bilayer of a coarse-grained lipid model at the Lβ′ phase. Two boundary lines were present in the initial configuration. It was shown that the lipid acyl tails became more disordered at the boundaries upon temperature increase.