Objective To study the temporal tendency and seasonal characteristics of the influenza activity intensity in northern provinces during 2006-2011.Methods The monthly intensity of influenza activity was estimated based on monthly influenza positive rate that was calculated from laboratory surveillance data in 15 northern provinces.High,medium and low intensity of influenza activity was defined when monthly influenza positive rate was ≥30％,10％ -30％ and ＜ 10％ in the month of influenza activity peak respectively.An influenza season was defined as the continuous months in which the high intensity was identified.Results In each of the consecutive 5 years,the low intensity was identified from June to July,the high intensity was identified from December to February of next year.In some years,the high or medium intensity observed from March to April,and from September to November.The influenza season lasted 4-7 month with the onset from September to December and the end from February to April.The peak month was in January in each of the other years except 2009 Influenza A (H1N1)pdm09 pandemic.The predominant circulating influenza strains varied by year.Conclusion A regular winter pattern is identified in the northern provinces.The influenza season occurred mainly from December to February of next year.Active influenza transmission continued from March to April,and from September to November.We recommend that annual vaccination with seasonal influenza vaccines in northern provinces should be carried out before December to improve effectiveness.