This experiment was designed to find potential mechanisms correlated with firmness increase by short-term treatment of high pressure CO2 in strawberries and to examine effects of preharvest treatment of chitosan with postharvest treatment of high pressure CO2 on storability of strawberry fruit. A short-term treatment of 100% CO2 increased strawberry fruit firmness both in vivo and in vitro. The increased firmness in response to CO2 treatment was maintained at significantly higher level than those of untreated fruit during storage. CO2 treatment induced the modification of pectin polymers, the decreased of water soluble pectins and increase of EDTA soluble ones. Also, CO2 treatment enhanced calcium binding to wall polymers, resulting in the increase of EDTA soluble pectins. Pectate lyase activity, an important factor for tissue softening, was significantly decreased by high pressure CO2 treatment. These results suggested that fruit firmness increase of strawberry by high pressure CO2 treatment derived from stimulation of calcium binding to pectin polymers. Preharvest Ca-chitosan spray was positive to the improvement of strawberry fruit quality such as delay of skin color changes during storage. When combined with postharvest treatment of high pressure CO2, it was more effective on firmness increase and maintenance. Preharvest Ca-chitosan spray was more effective on inhibition of postharvest skin disorder and decay. In ‘Seolhyang’ strawberry, the combined treatment of Ca-chitosan and high pressure CO2 further increased not only fruit firmness but residual effect of CO2 treatment. However, these effects were reduced when fruit were harvested late in the season. In conclusion, it expected that the combined treatment of preharvest Ca-chitosan spray and postharvest treatment of high pressure CO2 was effective postharvest technology to improve the overall fruit quality and storability such as firmness increase and decay control through fruit coating.