Cheeka Peak is a unique site for monitoring the background chemistry and aerosol contents of pristine marine air at mid-latitude. During long-range onshore winds that occur frequently throughout the year, it is predicted to have the cleanest air in the northern hemisphere. Measurements of CO and O3 were conducted at Cheeka Peak Observatory(CPO) on the northwestern tip of Washington state, USA during March 6 ～May 29, 2001. The data have been segregated to quantify the mixing ratio of these species in the Pacific marine atmosphere. Also the marine air masses were further classified into four categories based on 10-day backward isentropic trajectories; high, mid, and low latitude and those which had crossed over the Asian industrial region. The diurnal variation of CO and O3 at CPO showed a similar to tendency of background measurement site. When marine air mass flowed to CPO, CO concentration was lower and O3 was similar or higher than those of total data. The westerly flow from ocean, not easterly from continent occurred the high concentration of CO and O3 at CPO. Using the trajectory segregation of marine air mass, the comparison of concentration according to latitude calculated. the CO concentration of Asian trajectory was lower than other latitudes, O3 concentration was higher.