Resonantly stabilized free radicals (RSFRs) are contemplated as the reactive intermediates in molecular mass growth processes leading to polycyclic aromatic hydrocarbons (PAHs) which are prevalent in deep space and on Earth. Self-reaction routes of two RSFRs have been recognized as the fundamental but more efficient pathways to form fused benzenoid rings. Exploiting a chemical microreactor in combination with an isomer-selective identification technique through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high-resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), the present experiment provides compelling evidence on the formation of phenanthrene and minor anthracene with the presence of fulvenallenyl (C7H5•). Further theoretical calculations on the potential energy surfaces of C14H10 and C14H11 reveal that phenanthrene and anthracene can be efficiently produced via a hydrogen-assisted multi-step mechanism [C7H5•+C7H5•→i3(3,4-di(cyclopenta-2,4-dien-1-ylidene)cyclobut-1-ene); i3+H→ phenanthrene+H/anthracene+H or i3+H→i8(1-(cyclopenta-2,4-dien-1-ylidene)indene)+H→phenanthrene+H/anthracene+H] at low pressures, rather than through the one-step recombination-isomerization of fulvenallenyl radicals. This study provides a novel growth mechanism of tricyclic PAHs, especially in hydrogen-rich environment like combustion and interstellar ones, which advances the knowledge of PAH propagation and even the formation proceedings of carbonaceous nanoparticles in our universe.