A semiempirical method, the PM3-family-correlation (PM3-FC) method, has been developed to estimate the heats of formation (Delta H degrees(f)) of hydrocarbon free radicals. The PMS-FC method combines semiempirical PM3 calculations and family correlation with experimental data. The Delta H degrees(f) values for a test set of 67 hydrocarbon free radicals, including alkyl, cycloalkyl, alken-alpha-ly, cycloalken-alpha-yl, arylalk-alpha-ly, alkyn-alpha-yl, and vinyl radicals, were calculated by using the PM3 method and then correlated systematically with the experimental values from the literature. According to the structural analogy of the radicals and the observed correlation, the 67 radicals in the test set can be separated into five groups. The MP3-calculated and experimental values of the radicals in each group exhibit a very good linear correlation Using the obtained regression parameters to scale the PM3-calculated Delta H degrees(f) values leads to a very significant increase in estimation accuracy. For the test set of radicals, excluding the three cyclic hydrocarbon radicals with three carbon atoms in the ring and the three methyl radicals coordinated with three unsaturated groups, the average absolute deviation and the standard deviation between the PM3-FC estimated and experimental enthalpy values are 0.8 and 1.1 kcal/mol, respectively. The 95% confidence limit of the deviation between the PM3-FC-estimated and experimental values is in the interval of +/-2.1 kcal/mol. The error analysis of the PM3-FC method was preformed on the basis of statistical analysis by comparison with other estimation methods.