Using numerical simulations, the effect of the compressibility of air on the flow pattern of particles and pressure drop in the presence of particles during horizontal pneumatic transport operating under negative pressure was examined. The length and inside diameter of the pipeline were 30 m and 40 mm, respectively, and the chosen particles (4 mm in diameter) had densities of rho(p) = 1000 and 2000 kg/m(3). The mean air velocites at pipe the inlet were U-inlet = 19, 22, and 28 m/s, and the range of the mass flow rate ratios of particle to air, mu, was varied up to 2.0. For a given inlet air velocity, the difference in the flow pattern between compressible and incompressible flow calculation flow calculation is generally small. For rho(p) = 1000 kg/m(3) particles the additional pressure drop in compressible flow increases when mu is above 0.5 and U-inlet is 28 m/s, mu is above 1.3 and U-inlet is 22 m/s, and mu is above 1.5 and U-inlet is 19 m/s. In these cases, the particle flow pattern is homogeneous. For rho(p) = 2000 kg/m(3) particles, the pressure drop increases only when mu is above 1.5 and U-inlet is 28 m/s. The difference is not noticeable when the particle flow pattern is heterogeneous. Also, the difference in the additional pressure drop is much larger during homogeneous flow than heterogeneous flow.