Considering the necessity of achieving sustainability in cities, this research aims at assessing the practicality of the CASBEE-City tool for prioritizing development measures and strategies in Tehran Megalopolis, Iran. CASBEE is a method of assessing and rating the environmental performance of a built environment according to the Built Environment Efficiency (BEE) as an assessment index, which is based on the idea of eco-efficiency. This tool provides a composite index (BEE) which is calculated based on the city quality (Q) indices, including environmental, social, and economic aspects and the environmental load (L) covering the annual CO2-eq. emission per capita. The results of applying this tool to Tehran City show that this capital is ranked fairly poor (B-) in 2019, representing low quality and high load. According to the findings, recycling resources, social services, and public transportation, along with the decrease in carbon emissions are to be the priorities in the future city development plans. Presenting the results separated by each index, planners and decision-makers can accurately determine which aspect of the city’s quality should be given precedence for realizing sustainability in the mid and long term. A Comprehensive city assessment from the perspective of environmental efficiency is important because it provides guidelines for deciding upon a preferable course of action. This type of assessment should benefit not only citizens and local governments but also national governments. This kind of assessment helps national governments to determine which area should be the focus of further efforts toward improvement.