With a rigid requirement for environment protection, governments need to make appropriate policies to induce firms to adopt green technology in consideration of the rapidly increasing demand for environmentally friendly products. We investigated the government policy from the perspective of a supply chain, which consisted of the upstream government (she) and the downstream manufacturing firm (he). The government decided on the policy (tax or subsidy) to maximize the social welfare, while the firm decided on the greenness level of the product, which affects the consumers' choice behavior and hence his own demand. Assuming else being equal, the government should adopt the tax policy if consumers are very sensitive to the greenness, the cost of greening is high, or the negative impact due to carbon emission is large, and subsidize the firm otherwise. We also conduct some numerical studies when price is endogenous. The main insights can be carried over.Sustainability 2020, 12, 1548 2 of 13 $5.50 per 0.1 mpg for each product. After Finland and Sweden introduced a carbon dioxide tax in 1990s, carbon pricing systems, including taxes and cap-and-trade measures (which work similarly), have been adopted or planned in 40 countries and more than 20 cities, according to the World Bank. In Canada, carbon emissions in British Columbia dropped between 5% and 15% because of the tax [8]. Therefore, how to make appropriate policy to reduce carbon emissions is a big challenge facing governments.We investigated this problem from a supply chain perspective. Specifically, we considered a two-tier supply chain consisting of one government policy-maker and one manufacturer. The key questions we aimed to answer included: (i) Which policy (tax or subsidy) should be adopted by the government? (ii) How will different factors (e.g., environment externality, the cost of new technology, etc.) affect the policy making? (iii) How will the manufacturer react in response to the government policy?We established a stylized model setting to investigate the above questions. The government, as the Stackelberg leader, decides whether to tax or subsidize the firm in a particular industry, considering his possible reactions to different policies. The firm, as the Stackelberg follower, decides greening degree of his product, considering the consciousness of consumers to environmental protection (e.g., carbon emissions, pollutants, etc.). Our main findings are three-fold: Firstly, the government should adopt a tax policy if consumers are rather sensitive to the greenness level of products, the cost of greening is high, or the negative impact due to carbon emission is large, and subsidize the firm otherwise. Secondly, the firm will adopt a greener technology no matter the which policy (tax or subsidy) the government adopts compared with the situation without any government intervention. Lastly, the less sensitive consumers are toward the greenness of products, or the lower the cost of greening, the higher the consumers' surplus.In Section 2, we discuss the bo...