The Sierpiński triangle (ST) is a well-known fractal structure. Synthesis of stable molecular STs with robust covalent linkages is attractive but challenging. Here, we demonstrate the formation of a series of high-quality covalent STs via the on-surface dehydration reaction of 1,3-benzenediboronic acid with the presence of water as an equilibrium regulator at ambient atmosphere. Extended molecular fractals up to third generation are obtained, as disclosed by scanning tunnel microscope. The covalent STs show intriguing bright and dark contrasts irrespective of the fractal generations, which is related with epitaxial relationship of fractal structure to the underlying graphite lattice, as supported by theoretical simulations.