By a reaction of trimethylphosphine with an open-[60]fullerene, corresponding dimers could be generated via two-fold deoxygenation processes even though the formation of β-oxo-phosphorous ylide is inevitable, a part of which is hydrolyzed to yield an α-methylene carbonyl derivative. Nevertheless, Wittig reaction and aldol condensation did not proceed well, indicating the presence of an unknown dimerization pathway. In the ylide formation, 1-phosphonium-3-carbabetaine was previously proposed as a key intermediate. Upon assuming that the betaine also participates in the dimerization process, we examined a possible reaction pathway computationally. As the results, the betaine formed by a reaction with the first phosphine was suggested to undergo nucleophilic addition to an unreacted molecule of the open-[60]fullerene, yielding an epoxide dimer which is then deoxygenated by the second phosphine to furnish the desired open-[60]fullerene dimer.