Natural convection heat transfer for laminar air flow in a vertical circular pipe by using the boundary condition of constant wall heat flux in the ranges of Ra-L from 1.1 x 10(9) to 4.7 x 10(9) has been experimentally investigated. The experimental set up was designed for determining the effect of different configurations of restriction placed in the bottom position at the entry of the heated pipe on the surface temperature distribution, the local and average heat transfer coefficients. The apparatus was made from an electrically heated cylinder of a length 900 min and inside diameter 30 mm. The inlet configurations included a circular pipe of the same diameter as the heated pipe but with lengths of 60 cm, 120 cm, sharp edge and bell mouth. It was found that the surface temperature along the pipe surface for the same heat flux would be higher for a circular restriction with length of 120 cm and would be smaller for a bell mouth restriction. The results show that the local Nusselt number Nu and average Nusselt number (Nu(L)) are higher for a bell mouth restriction and smaller for the 120 cm restriction. For all the inlet restrictions configurations, the results show that the Nusselt number values increases as the heat flux increases. Empirical correlations were proposed in the form of LogNu(L) versus LogRa(L) for each.case investigated and a general correlation was obtained for all cases, which reveals the effect of the restriction existence on the natural convection heat transfer process in a vertical circular pipe. (C) 2006 Elsevier Ltd. All rights reserved.