Many abiotic factors were suggested to influence the haul-out of harbor seals (Phoca vitulina) but which factor is most influential to this behavior is still controversial. I recorded the number of haul seals which haul out at the beach of Hopkins Marine Reserve of Monterey Bay, California. Correlation analyses with various physical factors such as tides, wave height, and temperatures were conducted. Unlike the previous results, wave height was the most influential to the haul-out behavior. This implicates that strong wave actions delimit free swimming and foraging of harbor seals and lead them to choose to stay in their resting sites for later foraging.