An ultrasound contrast agent (UCA) plays a key role in ultrasound imaging to precisely diagnose coronary heart disease. Microcapsules composed of inner gas and shell materials are most commonly employed as ultrasound contrast agents. The ultrasonic properties of the microcapsules are significantly dependent on their size and size distribution. Herein, we prepared several uniform sized biodegradable polylactone microcapsules by combining a premix membrane emulsification technique and W/O/W method. We investigated various size-dependent factors to optimize the size and size distribution of the microcapsules. After evaluation for ultrasound imaging, ∼4 μm PLLA (poly(L-lactide)) microcapsules generated more intense ultrasound signals than PLGA7030 or PLGA5050 (poly(lactic-co-glycolic acid), the molar ratio of lactic acid and glycolic acid being 70 : 30 or 50 : 50), PEG-b-PLGA7030 (poly(ethylene glycol)-block-poly(lactic-co-glycolic acid)), PLC5050 (poly(L-lactide-co-caprolactone), the molar ratio of lactide and caprolactone being 50 : 50) and PEG-b-PLLA microcapsules. The signal duration of the PLLA and PEG-PLLA microcapsules could reach ca. 3 and 3.5 min continuously. The ultrasound signal intensity and duration of the signals of PLLA microcapsules were considerably stronger and longer than those of commercially available UCAs, showing that the PLLA microcapsules have a great potential as more efficient UCAs for biomedical imaging.