Household is the key factor for understanding the relationship between energy consumption and greenhouse emission. As one of the most important demographic factors, household size,not only affecting the consumption level,but also the consumption structure, plays a critical role in the household consumption. Due to the difference of carbon intensity between different goods, there are close relationships between household size,consumption level and structure, and carbon emission level and structure. Despite that importance, most of the professional and scholarly literature on greenhouse emission has neglected the effect of household size on the energy consumption structure and carbon emission. Based on data from China Family Panel Studies 2010,the article estimates economies of scale of different commodities and the consumption curve, the corresponding carbon emission curve with respect to household size, and reveals household size' s influence on the consumption structure and carbon emissions. The conclusion is that, firstly, all goods have economy of scale on household size, but there is a strong negative correlation between capita consumption and household size. The marginal utility of scale of economies is monotone decreasing, and the households with 2 to 4 members are most significantly affected by the economy of scale. Secondly, household size has different effects on the proportion of different goods, so the households with different size have different consumption structure, thereby different carbon levels and structures. Thirdly, the impact of economy of scale on carbon emission is non monotone decreasing. Based on above analysis, the article argues that building up ecological civilization and dealing with climate change require paying more attention to household characters. Especially, it is necessary to take full consideration of the change on household size when forecasting the trends of energy consumption and carbon emission. Meanwhile, different consumption policies depending on size and structure of household will help to guide rational consumption of energy and reduce the energy-saving emission.