The article explores the issue of G20 legitimacy, its role in facilitating G20 effectiveness and possible ways of improving G20 effectiveness in the deficiency of legitimacy. The legitimacy of G20 at this stage is not an issue. States representing a major share of world GDP and international financial transactions are expected to undertake all available steps to ensure sustainable growth of the world economy, finance and trade. The inevitable legitimacy problems must be analysed timely and adequately to avoid future crises and catastrophic shocks. The publication is prepared within the framework of a joint project of Russian International Affairs Council (RIAC) Project and International Organisations Research Institute of the NRU HSE "Enhancing Effectiveness of Russia's Participation in G8, G20 and BRICS in accordance with Russian Priorities and National Interests" in 2011.