Left parties in Europe are undertaking a green transformation.”Red-green” political ideology and parities’ environmental concerns are important symbols for this transformation. My research tries to explain why left parties take up environmental issues and what is the driving force of their concerns for a justice and low-carbon society. Ideology dimension, competiveness of national election, the severity of pollution and climate change are important factors that account for this change. Moreover, this research will focus on different energy policies and other environmental policies that taken by green left parties at national level as well as EU level. Due to institutional differences, divergent energy policies and green growth policies can be found in different left green parties.