A wide range of microfluidic devices for single stream focusing of cells and particles has emerged in recent years, based on both passive and active methods. Inertial microfluidics offers an attractive alternative to these methods, providing efficient and sheathless passive focusing of cells and beads. Nevertheless, in rectangular microchannels, the presence of multiple equilibrium positions necessitates complicated solutions involving manipulation of the 3D structure in order to achieve single stream flows. Here, we present a new approach to single-stream inertial focusing based on a triangular microchannel geometry. Changing the channel cross-sectional shape leads to asymmetry in the velocity profile, resulting in a size-dependent single stable equilibrium position near the channel apex. We demonstrate that soft lithography masters for such microchannels can be fabricated using PMMA through micromilling, and 15 μm diameter beads can be efficiently focused into a single stream. Confocal microscopy was used to confirm the focusing positions in the microchannel cross-section. We further integrated this device with a laser counting system to form a sheathless flow cytometer and demonstrated the counting of beads with an ∼326 s -1 throughput. The use of a triangular cross-section offers a number of benefits, including simplicity of the fundamental principle and geometry, control of design, a small footprint, and ease of integration, as well as high-precision single position focusing.