For the past few decades, urbanization has been occurring at a rapid pace. The relationship between urban density and energy consumption, which are accompanied by emissions of greenhouse gases, is still not conclusive. This study examined the relationship between urban form and carbon dioxide (CO2) emissions from urban areas in 50 cities in Japan. We employed satellite imagery to delineate urban areas. The maps of administrative boundary were used to clip urban regions from each scene of satellite image. The clipped images were classified into a binary class: urban built-up and others. The sectoral data for the CO2 emissions at municipal level in 2005 was obtained from various sources. We examined two types of approaches to quantify urban forms. One method involved landscape metrics, which included two types of metrics, compactness and complexity. The other method is a new method and it relies on quantifying the reduction rate of urban areas from the city center by applying ring-shaped buffers. We called this method the buffer compactness index. Results indicated that there are correlations between several indices of urban forms and sectoral CO2 emissions.