Higher-harmonic ac susceptibility was investigated for single-crystal YBa₂Cu₃O_7-x thin films, when a magnetic field was applied perpendicular to the film plane. Real and imaginary part of the harmonic susceptibility were successfully measured independently each other. Odd terms had finite values and varied complicatedly in the temperature region when the imaginary part of fundamental susceptibility formed a single peak. Second harmonic also had a finite value, however, its value was extremely small. These results suggest the existence of some mechanism to obstruct the vortex motion in the specimen.