For horizontal circular pipes under uniform wall heat flux boundary condition and three different inlet configurations (reentrant, square-edged, bell-mouth), Ghajar and Tam (1995) developed flow regime maps for the determination of the boundary between single-phase forced and mixed convection using experimental data of Ghajar and Tam (1994). Based on the ratio of the local peripheral heat transfer coefficient at the top and the bottom, the heat transfer data was classified as either forced or mixed convection among the different flow regimes. The forced-mixed convection boundary was then obtained by empirical correlations. From the flow maps, heat transfer correlations for different flow regimes were recommended. Recently Trafalis et al. (2005) used the Multi-class Support Vector Machines (SVM) method to classify vertical and horizontal two-phase flow regimes in 4 pipes with good accuracy. In this study, the SVM method was applied to the single-phase experimental data of Ghajar and Tam (1994) and new flow regime maps were developed. Five flow regimes (forced turbulent, forced transition, mixed transition, forced laminar, mixed laminar) were identified in the flow maps using Reynolds and Rayleigh numbers as the identifying parameters. The flow regimes on the boundaries of the new maps were represented by the SVM decision functions. The results show that the new flow regime maps for the three types of inlets can classify the forced and mixed convection experimental data in different flow regimes with good accuracy.