Carbon/nylon composites were ultrasonically welded under controlled welding pressure and time. The optimized pressure and time for the highest joining strength are found by conducting the lap shear test. The acoustic emission (AE) technique used during the test is able to detect the first-damage load (FDL) and identify the damage mechanisms. Fiber breakage contributes to higher lap shear strength of the specimen, while debonding and pull-out lead to lower strength. Furthermore, the configurations of the AE curves provide the judgment of the magnitude of lap shear strengths of composites. The moisture absorption in welded composites follows Fick's law of diffusion; and lap shear strength of the composite decreases with increasing moisture content, the correlation of which follows an exponential decay function. The reduction of the strength is due to weaker hydrogen bond in the matrix connected between water and amide groups, and damage caused by the swelling of the matrix.