Full three dimensional Particle‐in‐Cell (PIC) simulations are performed in order to investigate effects of field‐aligned (parallel) ion flow shears on low‐frequency plasma instabilities. It is shown that the parallel ion flow velocity shear can induce the ion‐acoustic instability, even when the ion flow velocity is so small that the instability can not take place. Simulation results are consistent with the analysis based on the local theory. © 2004 American Institute of Physics