An after treatment system (ATS) for a vehicle having an ATS module includes, fluidly connected in series, an inlet, a Diesel Oxidation Catalysts (DOC), a urea mixer and a Selective Catalytic Reduction (SCR), and an outlet. The inlet is fluidly connected to an output of an engine of the vehicle and the outlet is fluidly connected to an outlet tube of the vehicle. The inlet, DOC, mixer, SCR and outlet are arranged to define a substantial rectangular path of a flow (F) of exhaust gases flowing in the ATS, with the inlet and the outlet being positioned at a same vertex of the substantial rectangular path of the flow (F).