Understanding the intersystem crossing (ISC) mechanism of organic compounds is essential for designing new triplet photosensitizers. We investigated the ISC mechanism of a heavy atom-free Bodipy derivative with thiomethyl substitution (S-BDP). A long-lived triplet state was observed with nanosecond transient absorption spectroscopy with lifetime of 7.5 ms in a polymer film and 178 ms in fluid solution, longer as compared with what was previously reported . Femtosecond transient absorption studies retrieved an ISC time constant of ~3 ns. Time-resolved electron paramagnetic resonance (TREPR) indicated a special triplet electron spin polarization phase (ESP) pattern (a, e, a, e, a, e), different from the typical ESP (e, e, e, a, a, a) for the spin-orbit coupling mechanism. This indicates that the electron spin selectivity of the ISC of S-BDP is different from the normal SOC effect in iodo-Bodipy. Simulations of the TREPR spectra give a zero-field-splitting D parameter of -2257 MHz, much smaller as compared to the reference 2,6-diiodo-Bodipy (D = -4380 MHz). The computed SOC matrix elements (0.28-1.59 cm-1) and energy gaps for the S1/Tn states suggest that the energy matching between the S1 and T2/T3 states (supported by the largest kISC ~109 s-1) enhances the ISC for this compound.