Metal-coordinated por-phyrin and related compounds are important for developing, molecular architectures that mimic enzymes. Porphycene, a structural isomer of porphyrin, has shown unique properties in semiartificial myoglobin. To explore its potential as a molecular building block, we studied the molecular assembly of nickel(II) tetra(n-propyl)porphycene (NiTPrPc), a metalloporphycene with introduced tetra n-propyl moieties, on the Au(111) electrode surface using in situ scanning tunneling microscopy. Because of the low molecular symmetry of NiTPrPc, the molecular assembly undergoes unique phase transitions due to conformational change of the n-propyl moieties. The phase transitions can be precisely controlled by the electrode potential, demonstrating that the latter can play an important role in the porphycene molecular assembly on Au surface. This new discovery indicates possible uses of this porphycene framework in molecular engineering.