The combined effects of using cinnamaldehyde with catechin, quercetin or eugenol against wood decay fungi were examined by comparing their isoeffective concentrations to that of individual compound. Among all combinations, cinnamaldehyde with eugenol revealed the strongest synergy against Laetiporus sulphureus. The synergism was due to the interference of fungal cell wall synthesis and cell wall destruction plus radical scavenging effect. Results also suggested that antioxidant with fungicidal effect might be a better candidate than pure antioxidant for the system of fungicide/antioxidant.