The spatial structure and temporal evolution of the intraseasonal oscillation (ISO) in boreal summer over the midlatitudeNorth PacificOcean are investigated, through the diagnosis of NCEP reanalysis data. It is found that the midlatitude ISO has an equivalent-barotropic structure, with maximum amplitude at 250 hPa. Initiated near 1208W, the ISO perturbation propagates westward at a phase speed of about 2.4 m s and reaches a maximum amplitude at 1508W. A diagnosis of barotropic energy conversion shows that the ISO gains energy from the summer mean flow in the ISO activity region. A center-followed column-averaged vorticity budget analysis shows that the nonlinear eddy meridional vorticity transport plays a major role in the growth of the ISO perturbation. There is a two-way interaction between ISO flows and synoptic eddies.While a cyclonic (anticyclonic) ISOflowcauses synoptic-scale eddies to tilt toward the northwest–southeast (northeast–southwest) direction, the tilted synoptic eddies then exert a positive feedback to reinforce the ISO cyclonic (anticyclonic) flow through eddy vorticity transport. The reanalysis data and numerical simulations show that the midlatitude ISO is primarily driven by local processes and the tropical forcing accounts for about 20% of total intraseasonal variability inmidlatitudes.However, 20%might be an underestimate given that the tropical intraseasonal forcing is not fully included in the current observational analysis and modeling experiment.