The combustion has long been applied for industrial synthesis of carbon materials such as fullerenes as well as carbon particles (known as carbon black), but the components and structures of the carbon soot are far from being clarified. Herein, we retrieve an unprecedented hydrofullerene C66H4 from a soot of a low-pressure combustion of benzene-acetylene-oxygen. Unambiguously characterized by single-crystal X-ray diffraction, the C66H4 renders a nonclassical geometry incorporating two heptagons and two pairs of fused pentagons in a C-2v symmetry. The common vertexes of the fused pentagons are bonded with four hydrogen atoms to convert the hydrogen-linking carbon atoms from sp(2) to sp(3) hybridization, which together with the adjacent heptagons essentially releases the sp(2)-bond strains on the abutting-pentagon sites of the diheptagonal fused pentagon C-66 (dihept-C-66). DFT computations suggest the possibility for an in situ hydrogenation process leading to stabilization of the dihept-C-66. In addition, the experiments have been carried out to study heptagon-dependent properties of dihept-C66H4, indicating the key responsibility of the heptagon for changing hydrocarbon activity and electronic properties. The present work with the unprecedented double heptagon -containing hydrofullerene successfully isolated and identified as one of the low-pressure combustion products shows that the heptagon is a new building block for constructing fullerene products in addition to pentagons and hexagons in low-pressure combustion systems.