Closed-cell polyimide rigid foams with different Calculated Molecular Weight (Calc'd M-n) and foam density () have been prepared by thermal foaming of nadimide-endcapped imideoligomers (NAIO) powder. The NAIO powder was obtained by thermally treating of a PMR poly(amide ester) solution derived from the reaction of diethyl ester of 2,3,3,4-biphenyltetracarboxylic dianhydride (-BPDE) and p-phenylenediamine (p-PDA) using monoethyl ester of cis-5-norbornene- endo-2,3-dicarboxylic acid (NE) as reactive endcapping agent in ethyl alcohol. Effect of Calc'd M-n and foam density () on mechanical and thermal properties of the polyimide rigid foams have been systematically investigated. It was found that the thermal foaming properties of NAIO powders were affected by the Calc'd M-n. The appreciate Calc'd M-n could yield polyimide foams with both high closed-cell content (C-c) (>80%) and outstanding mechanical properties. Moreover, the thermal properties were reduced by increasing of Calc'd M-n and the mechanical properties improved gradually by increasing foam densities. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3282-3291, 2013