When bulk solid materials are transported in conventional pneumatic conveying systems, three flow modes have been observed. These are: (i) smooth transition from dilute- to fluidised dense-phase, (ii) dilute-phase, unstable-zone and slug-flow and (iii) dilute-phase only. The flow mode for a bulk solid material is totally dependant on material properties, in particular, those properties which involve particle/air interaction (e.g., permeability, air retention and de-aeration). Usually, the particle/air interaction characteristics are a function of basic particle properties, such as particle size, size distribution, density and shape. Loose-poured bulk density also is a function of these basic particle properties. Therefore, there should be a relationship between the loose-poured bulk density and the particle/air interaction characteristics. Based on experimental results and theoretical analysis, a new flow mode diagram is developed for the purpose of selecting suitable flow mode for a particular material. Based on the developed flow mode diagram, the bulk solid materials can be classified into three groups (PC1, PC2 and PC3), characterised simply by loose poured bulk density and median particle diameter. Materials in,group PCI can be transported smoothly and gently from dilute- to fluidised dense-phase. Materials in group PC2 can be transported in dilute-phase, unstable zone or slug-flow and materials in group PC3 are conveyed in dilute-phase only. A good accuracy is achieved when many test results with the observed flow modes are superimposed on the developed flow mode diagram.