The purpose of this study is to propose new thermal index for outdoor and non-uniform environments with heat conduction, such as when a person sits on a bench at a park. This paper describes mathematically the theory of how solar radiation and heat conduction, as well as air temperature, humidity, air velocity and longwave radiation, are incorporated into the new index and how these thermal factors that may not be uniform are treated. Another important feature is that separate indices are generated for each factor while the new index is derived, It is expected that the new index will help us to understand how much each factor affects the human thermal comfort in outdoor and non-uniform environments with heat conduction. (C) 2011 Elsevier B.V. All rights reserved.