Reactive blending, called also as in-situ compatibilization or reactive compatibilization, involving interfacial reactions leading to the in-situ formation of graft or block copolymer as compatibizers is an efficient route to create new materials that combines the desirable properties of each blend composition. It is very challenging to investigate a reactive blending process because both mixing and interfacial reactions between reactive polymers are highly coupled. Most importantly, the amount of the in-situ formed copolymer is often so low that it is very difficult to determine it in a relatively accurate manner. For this reason, a new concept called reactive compatibilizer-tracer, which bears reactive groups capable of reacting with its counterpart upon forming a copolymer for in-situ compatibilization of a reactive polymer blend and at the same time fluorescent labels allowing determining very small amounts of the in-situ formed compatibilizer was developed in this work. A copolymer of styrene (St) and 3-isopropenyl-α,α’-dimethylbenzene isocyanate (TMI) (PS-co-TMI) with fluorescent group as a reactive compatibilizer-tracer for the polymer blend composed of polystyrene (PS) as a matrix and polyamide (PA6) as a dispersed phase was used to study their interfacial reaction and morphology development. Contrary to common wisdom, although the interfacial reaction still increases, the dispersed phase domain size first decreases and then increases as mixing time. It is attributed to the two roles mixing played in the reactive polymer blend. In the early stage of mixing, mixing increases the collision between two reactive polymers which is useful for the morphology development. However, the in-situ formed copolymer is only physically held at the interface. In the latter period of mixing, mixing could pull the in-situ copolymer out of the interface, leading to the in-situ formed copolymer losing its efficiency and the coalescence of the dispersed phase.