Dimer acid (DA)-based nylon 636/nylon 66 copolymers were synthesized by in situ polymerization. The effects of incorporating nylon 66 on the mechanical and thermal properties were characterized by means of intrinsic viscosity determination, attenuated total reflection (ATR)–Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry, thermogravimetric analysis, and mechanical testing. The results show that the intrinsic viscosity of the copolymers ranged from 1.0 to 2.1 dL/g, depending on the content of nylon 66. The incorporation of nylon 66 into DA-based nylon 636 had no significant effect on the values of the glass-transition temperature, melting temperature, temperature at 50% weight loss, or temperature at the maximum rate of decomposition, but the crystallization temperature, crystallinity degree, and extrapolated onset temperature increased. ATR–FTIR spectroscopy and XRD demonstrated that with increasing nylon 66 mass, the content of α- and β-crystal forms would increase accordingly. The mechanical test data revealed that with increasing nylon 66 concentration, the tensile strength of the copolymers increased, and the flexural strength and flexural modulus first increased and then decreased. However, the notched Izod impact strength decreased. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39845.