Abstract: A mixing chamber assembly adapted for an engine aftertreatment system includes a first sub-compartment, a second sub-compartment, a mounting hole for installing a urea injector for spraying urea droplets into the first sub-compartment, and a middle sub-compartment connecting the first sub-compartment and the second sub-compartment. A mixer is positioned in the middle sub-compartment and includes a plurality of first blades inclined to the second sub-compartment. The first blades are divided into two groups of different inclined directions in order to achieve a double-swirl mixing effect. A plurality of second blades are inclined in the same direction and positioned upstream the first blades. As a result, flow distance within limited space can be increased to assure sufficient mixing of the urea droplets and the exhaust gas. The urea droplets are sufficiently heated to improve urea evaporation rate, improve uniformity of ammonia molecule and reduce the risk of urea deposits.