Although the triplet states of fullerenes have prosperous applications, it remains unclear how the structural parameters of singlet and triplet states control the intersystem crossing (ISC) rates. Here, electronic structure calculations (reorganization energy, driving force, and spin-orbit coupling) and a rate theory (Marcus formula) are employed to quantitatively predict the ISC rates of isolated fullerenes C-n (n = 60-110). The results demonstrate that the driving force is not the only factor to predict the ISC rates. For instance, although C-80, C-82, and C-110 have the favorable driving force, the ISC rates are close to zero because of small spin obit couplings, whereas small ISC rates of C-96 and C-100 result from quite small reorganization energies. Meanwhile, in addition to well-known C-60 and C-70, C-92 possesses good ISC property with obviously large ISC rate. C-92 also has a higher triplet-state energy than singlet-state oxygen energy; it may thus have a good photoactive property.