Using a long-distance microscope imaging system and a lock-in phase integrate technique, optical measurements demonstrate that the evolutional behaviors of a phospholipid-shelled micro-bubble, a kind of ultrasound contrast agent, under two ultrasound frequencies and different pressures. Dozens of frames of an oscillating microbubble for one acoustic cycle are captured. The experimental data of micro-bubble diameter are fit by the numerical calculations of the Hoff model and the Rayleigh-Plesset model, respectively. The results show the good agreement between experimental data and the theoretical calculation in the Hoff model. In addition, the spectral analysis based on the experimental data indicates that the relative intensity of the second harmonic increases with the driving pressure amplitude.