The quantum Monte Carlo (QMC) method is used to compute the atomization energy and the heat of formation of the propargyl radical, C3H3. The effective core potential and fixed-node approximations are used in the diffusion Monte Carlo (DMC) variant of QMC. Two generalized gradient approximation density functionals, B3LYP and B3PW91, are also applied for comparison. The atomization energy determined by these methods is 606.12 kcal/mol (B3LYP), 610.24 kcal/mol (B3PW91), and 607.6(0.6) (DMC). The latter compares favorably with separate measurements of 608.0(3.0) and 608.5(1.2) kcal/mol. The Delta H-f(298) determined by these methods is 84.03 kcal/mol (B3LYP), 79.91 kcal/mol (B3PW91), 82.5(0.6) (DMC), and two independent measurements yield values of 82.5(3.0) and 81.5(1.2) kcal/mol.