The gas phase ion-molecule reaction of propargylium (HCCCH2+) with acetylene (C2H2) to produce cyclopropenylium (c-C3H3+) with C2H2 has been investigated theoretically at the B3LYP/6-31G(d) and single-point QCISD/6-311G(d,p) levels. The detailed mechanism for the observed isotope exchange between HCCCH2+ and C2D2 has also discussed. Three intermediates 1 CH2CCH2CCH+, 2 H2C2. CHCCH2+ and 3 c-C4H3-CH2+ are shown to play important roles in the product formation and isotope exchange processes, rather than the low-lying isomers 4 c-C3H2-CH2+, 7 c-(CH)(5)(+) and 8 pyramidal C5H5+. Our calculated results agree well with the available experimental data and may be helpful for understanding the mechanism for combustion processes.