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Informal economy i.e. unrecorded economy, is one of the important problems in developing countries which affects the efficiency of economic activities in formal sector. Informal economy is also an important source of air pollution. This paper aims at estimating informal economy in Iran over the period 1980-2009 based on the mount of CO2 emissions and the country forest areas and using Kalman Filter approach. The results indicate that: 1) there is a significant and long run relationship between CO2 emissions, the size of forest areas and firm’s industrial activities and total national product, 2) Total national product is more than recorded data in the study period so the existence of informal economy can’t be rejected during this period. 3) The average share of informal economy in total GDP is about 35.6 %.