An apparatus for aftertreatment of exhaust gas includes a housing configured to define an inner cavity; an exhaust inlet arranged to the housing for entering exhaust gas flow to the inner cavity; a mixer unit arranged in the inner cavity to dispense a reactant to the exhaust gas flow; a mixing device, located downstream of the mixer unit, to evenly mix the reactant to the exhaust gas flow. The mixing device includes: a toroidal cylinder to receive the exhaust gas flow from the mixer unit via a solid tubular element, wherein the solid tubular element is in a centric manner to a first base of the toroidal cylinder for guiding the exhaust gas flow led into the toroidal cylinder to swirl inside the toroidal cylinder; and an exhaust outlet arranged to a curved side of the toroidal cylinder, to exit the exhaust gas flow from the mixing device.