The potential energy surface of C 3H 3O +, including twenty-two isomers and thirty-four interconversion transition states, is investigated at the B3LYP/6-31G(d,p) and QCISD/6-31G(d,p) (single-point) levels. The energetic order and the thermal stability of C 3H 3O + isomers are determined. It is shown that five energetically low-lying C 3H 3O + isomers (a) CH 2CHCO +, (b) c-CHCHC–OH +, (c) trans-CHCCHOH +, (c′) cis-CHCCHOH +, and (d) CH 2CCOH + have high thermal stability both towards isomerization and dissociation, among which the O-protonated cyclopropenone isomer (b) lies in the deepest potential well. It is also shown that four high-lying isomers (i) CHCOCH 2 +, (n) c-COC–CH 3 +, (o) CHCCOH 2 +, and (t) CCCHOH 2 + have moderate thermal stability, while the remaining thirteen ones are thermally unstable either towards isomerization to more stable ones or dissociation into fragments. The calculated results may be helpful for identifying various C 3H 3O + isomers as well as their related deprotonated C 3H 2O isomers, especially the hitherto unknown isomer (b), as well as the deprotonated forms CH 2CCO (propadienone), c-CHCHC–O (cyclopropenone) and CCCHOH (hydroxypropadienylidene) which still remains astrophysically undetected. In addition, the results presented in this article may provide some useful information for discussing the combustion and interstellar chemistry in which the C 3H 3O + potential energy surface is involved.