A series of new poly(amide-imide)s were synthesized by direct polycondensation of the 1,6-bis(trimellitimido)diamantane (1) with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) containing lithium chloride. The poly(amide-imide)s had inherent viscosities of 0.33–1.55 dL/g. The new polycyclic monomer 1 was synthesized from 1,6-diaminodiamantane with trimellitic anhydride. Dynamic mechanical analysis revealed polymers 3a–c to have main transitions at 385°C, 385°C, and 330°C, respectively. Moreover, these polymers were stable at high temperatures and maintained good mechanical properties (G′ ≈ 108 Pa) up to temperatures close to the main transition well above 350°C. Their 5% weight loss temperatures ranged from 421 to 448°C in nitrogen, and from 417 to 426°C in air. A glass transition of poly(amide-imide)s 3 was not observed. Poly(amide-imide)s 3 (except 3e) were soluble in NMP, pyridine, and o-chlorophenol. Some of the polymers (3a–c) could be cast into flexible films. Their cast films had tensile strengths ranging from 35.5 to 46.9 MPa, elongations at break from 5.8 to 7.5%, and initial moduli from 1.8 to 2.1 GPa.