The wild populations of yew trees in northeastern China are rarely attacked by insects and diseases. To better understand its defense mechanism behind such phenomenon, potential antifeeding activity of two key secondary metabolic compounds of yew, cephalomannine and taxol, were tested against Lymantria dispar L. larvae in both “selective” and “non-selective” birch leaf disc bioassays. Our results showed that both cephalomannine and taxol exhibited significant antifeeding effect on L. dispar larvae. The antifeedancy of cephalomannine seemed to increase with its concentration in a dose-responsive fashion in both the “selective” and “non-selective” tests, reaching 67.46% and 72.37%, respectively, at 2 mg/mL. In contrast, taxol showed a very strong antifeeding effect even at the lowest concentration tested, reaching 87.18% of antifeedancy in “no-selective” test at 0.25 mg/mL. In the “selective” bioassay, the antifeeding activity of taxol seemed to decrease with the feeding time at 1 mg/mL or lower, but increased with the time at higher concentrations (1 mg/mL),reaching 74.95% after 48 h. In general, taxol showed stronger antifeeding activity against L. dispar larvae than did the cephalomannine. The significant antifeeding activity of the two metabolic compounds of yew, especially taxol, might partially explain why the yew trees are not or rarely attacked by insects.