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Note 6 - Condensed Income Statement of EXCOR, Tianjin Zerust, and All Other Joint Ventures (Details) - USD ($)
 3 Months Ended 9 Months Ended
May 31, 2016
May 31, 2015
May 31, 2016
May 31, 2015
EXCOR [Member]        
Net sales     $ 25,314,260 $ 27,580,051
Gross profit     13,512,404 15,113,100
Net income     5,409,407 6,334,009
Northern Technologies International Corporation’s share of equity in income of joint ventures     2,714,905 3,152,489
All Other [Member]        
Net sales     40,909,254 44,659,262
Gross profit     16,233,391 20,572,413
Net income     1,778,867 3,034,661
Northern Technologies International Corporation’s share of equity in income of joint ventures     885,979 1,520,934
Tianjin Zerust [Member]        
Net sales       3,735,457
Gross profit       1,783,673
Net income       265,648
Northern Technologies International Corporation’s share of equity in income of joint ventures       132,824
Net sales     66,223,514 75,974,770
Gross profit     29,745,795 37,469,186
Net income     7,188,274 9,634,318
Northern Technologies International Corporation’s share of equity in income of joint ventures $ 1,664,464 $ 1,724,915 $ 3,600,884 $ 4,806,247