Joining of Plastics is mainly done at the places where the product cannot be fabricated in a single shot manufacturing process. Plastic Welding Processes can be divided into 2 groups; one involves mechanical movement and the other requires external heating. In this study the vibration welding, which involves mechanical movement has been applied to join the two thermo plastics of different character. It is very easy to weld two crystalline plastics or two semi crystalline plastics but it is very difficult to join the amorphous thermoplastics either together or crystalline or semi crystalline plastics as another partner. In this case, either in-situ compatibilizer or externally applied compatibilizer is a must. We tried to join the PMMA-PBT system and PMMA- nylon 66 systems using various welding process parameters. Externally applied compatibilizers used are PMMA-PBT= 50-50%, 70-30%, 30-70%. Similar proportions were used for PMMA- Nylon-66 system. It has been observed that the viscosity play a vital role in vibration welding process which changes the process parameters in order to get the efficient welding. The various parameters like amplitude, welding pressure, Dwell Time and melt down depth (distance) were studied. It has also been found that in case of PMMA-PBT system the more welding pressure and higher amplitude is required. Melt down depth however is little bit lower may be because of higher viscosity of PBT. The weld line was characterized by FESEM and found to be folded down in the case of PMMA- Nylon-66 system. However for PBT system, the folding is in the same plane of weld line. Better weldability have been observed when the two types of externally applied compatibilizer are applied separately on to the two substrates and then vibration welded. Two types of solution compatibilizer in the ratio of 70-30% and 30-70% is the better option than the 50-50% variety. Keywords: Vibration welding, PMMA- PBT, PMMA-Nylon66, Welding Pressure, welding Amplitude.