The present work deals with the results of an experimental investigation on heat transfer in water cooled vertical pipes, for thermal-hydraulic conditions ranging from forced convective flow to mixed convective flow. The flow of water in the pipe is upwards. Experimental data confirm the reduction in the heat transfer rate for mixed convection in upward heat flow, mainly due to-the laminarization effect in the near-wall region (buoyancy effect). They are in a very good agreement with numerical methods, such as the k-epsilon-model. A new method for the calculation of the heat transfer coefficient in upward mixed convection heated flow is proposed : It is based on the well-known superposition method (heated downflow) modified-accounting for the-phenomenology of the upward heated flow in comparison with downflow heated conditions.