Forty-eight　daily　time　interval　PM2.5　samples　were　collected　from　December　2006　to　January　2008　in　an　urban　site　in　Shanghai,　China.　Concentrations　and　compositions　of　polycyclic　aromatic　hydrocarbons　(PAHs)　were　analyzed　with　GC-MS　to　study　the　diurnal　and　seasonal　variations　and　to　identify　the　main　emitting　sources.　The　diurnal　variation　of　the　PAHs　concentrations　was　greater　in　the　late　autumn　and　winter　sampling　days,　and　was　greatly　influenced　by　meteorological　conditions　such　as　wind　speed　and　ambient　temperature.　The　concentration　of　PAHs　in　the　mornings　(6:30-10:00)　increased　distinctly,　and　was　high　in　the　late　autumn　and　winter　sampling　days,　indicating　the　contribution　from　vehicle　emissions　during　rush　hours.　The　diurnal　variation　of　the　high　molecular　weight　PAHs　did　not　seem　to　be　controlled　by　the　shift　of　gas-particle　partitioning　due　to　temperature　variation,　instead,　it　could　be　indicative　of　the　variation　in　the　source.　Statistical　analyses　showed　that　the　concentrations　of　PAHs　were　negatively　correlated　with　temperature　and　wind　speed,　and　positively　correlated　with　relative　humidity.　Diagnostic　ratios　of　PAHs　suggested　mixed　emission　sources　of　petroleum　and　coal/biomass　combustion　for　PAHs　in　the　PM2.5　in　Shanghai.