Chichibabin's hydrocarbon and Muller's hydrocarbon are classical open-shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR-2 and OxR-3, based on the newly developed oxindolyl radical. X-ray crystallographic analysis on OxR-2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y0 = 11.1%) and large singlet-triplet gap (ΔES-T = -10.8 kcal/mol). Variable-temperature NMR studies on OxR-2 disclose a slow cis-/trans- isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔGǂ298K = 15~16 kcal/mol). OxR-3 exhibits a much larger diradical character (y0 = 80.6%) and a smaller singlet-triple gap (ΔES-T = -3.5 kcal/mol), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon-centered monoradical and diradicaloid.