Recently, interest in micro-jet research has increased that is related to the potential use of micro-jets in various applications of aerospace and chemical industries. Non-axisymmetric nozzles, in particular rectangular ones, are often used due to design considerations. The purpose of this work is numerical and experimental study of the ejection of underexpanded micro jets from a slotted nozzle into still air in wide range of jet pressure ratio, height of the rectangular nozzle, and height-to-width ratio. The jets have sonic velocity at the nozzle exit. The paper presents the results of experiments on visualization of the gas micro-jets outflow using a shadowgraph method, as well as longitudinal and transverse pressure distribution measured by Pitot tubes. Numerical simulation of the ejection of underexpanded microjets from the slotted nozzle into still air has been carried out in 2D statement. Calculations have been performed at the same conditions in the still area as experimental ones. Stationary Reynolds-averaged Navier-Stokes equations have been solved for the turbulent flow regime with the turbulence model k-ω SST. The measurements and visualization of the flow allowed us to obtain parameters: the length both of a non-dimensional average shock cell and the first one.