Proper dispersion should be guaranteed when using carbon nanotubes (CNTs) as effective absorbents. Physical dispersion methods, such as mechanical mixing, ball milling, and calendering process (three-roll milling), were adopted to disperse CNTs in the bismaleimide matrix to foam. The effects of the dispersion methods on viscosity, bubble morphology, and microwave-absorbing properties at 12–18 GHz were studied. The results indicated that, three-roll milling can more efficiently separate individual CNTs from the agglomerates compared with the other two methods. This method exhibited the highest viscosity, the most irregular cell shape, and the best microwave-absorbing property. The effects of CNT concentration and foam thickness on the microwave-absorbing properties were also studied. The reflectivity decreased and the position of the reflectivity peak moved to a lower value with increase in CNT concentration and foam thickness. The foam with 30 mm thickness achieved a reflection loss below −8 dB over 3.0 GHz in the range of 13.5–16.5 GHz. The minimum value is −14.2 dB at 14.6 GHz. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40233.