This paper treats of the laminar boundary layer of free convection along a vertical surface immersed in a temperature stratified environment. The condition of the existence of similarity solution is preliminarily discussed by reconsidering the definition of the coefficient of thermal expansion. A similarity solution for the case of uniform surface heat flux and solutions by the methods of perturbation and finite-difference approximation for the case of isothermal surface are presented. Measured profiles of temperature and velocity agree well with theoretical ones. The local temperature defect occurs in the case of relatively large Prandtl number, but no flow reversal occurs.