Polymer-polymer composite of nylon and poly(p-phenylene terephthalamide)(PPTA) was prepared by in-situ polymerization of p-phenylene diamine and terephthaloyl chloride in nylon solution of nylon terpolymer and N-methyl 2-pyrrolidone (NMP). For the PPTA/nylon composites, the shift of the glass transition temperature of nylon to a higher temperature was observed, which implied a molecular level mixing of PPTA in nylon matrix. The structure of nylon, PPTA and the composites were observed with X-ray diffractometer. The crystal structure of PPTA in the composite was unchanged although that of nylon was changed considerably by the addition of even 20% PTA. Electron microscopy revealed that microfibrils of PPTA were homogeneously dispersed in the nylon matrix. The large increase of the melt viscosity with the increase of PPTA content was also observed with rheometric dynamic spectrometer (RDS). The mechanical properties such as Young''s modulus and yield strength were remarkably improved with the addition of PPTA.