The viscoelasticity and subharmonic generation of a kind of lipid ultrasound contrast agent are investigated.Based on the measurement of the sound attenuation spectrum, the viscoelasticity of the lipid shell is estimated by use of an optimization method. Shear modulus Gs=10MPa and shear viscosity μs=1.49N·S/m2 are obtained. The nonlinear oscillation of the encapsulated microbubble is studied with Church’s model theoretically and experimentally.Especially, the dependence of subharmonic on the incident acoustic pressure is studied. The results reveal that the development of the subharmonic undergoes three stages, i.e. occurrence, growth and saturation, and that hysteresis appears in descending ramp insonation.