[en] The excitation functions of heavy residues, representing complete and incomplete fusion products, produced in the reaction of ¹²C and ¹³C on ¹⁸¹Ta have been measured over the projectile energy range of 5 to 6.5 MeV/nucleon by the recoil catcher method and off-line γ-ray spectrometry. Comparison of the measured excitation functions with those calculated using the PACE2 code based on the statistical model revealed the occurrence of incomplete fusion reactions in the formation of α emission products. The fraction of incomplete fusion cross sections in the ¹²C + ¹⁸¹Ta reaction was found to be higher, by a factor of ∼2, than that in the ¹³C + ¹⁸¹Ta reaction. The results have been discussed in terms of the effect of alpha cluster structure of the projectile on incomplete fusion reactions