New poly(urethane-imide)s (PUIMs/MWCNT) nanocomposites (8a & 8b) containing two different amounts of MWCNTs were prepared by in situ polymerization technique. We first prepared a new diisocynate (5) containing imide moiety by a proper synthetic method. Then new poly(urethane-imide) (7) was prepared by reaction of diisocynate (5) with hydroquinone and structure of resulting polymer was characterized by mean of FT-IR spectroscopy and elemental analysis. Also inherent viscosity of resulting poly(urethane-imide) (7) was measured about 0.35 dL/g. Transmission electron microscopy (TEM) showed that MWCNTs were exfoliated in the polymer matrix, resulting in well-dispersed morphologies at 1.5 and 2.5 mass% MWCNT contents. The effects of multiwall carbon nanotubes (MWCNT) on the thermal property of new PUIMs/MWCNT derived from diisocynate (5) were investigated by thermogravimetric analysis (TGA) in nitrogen atmosphere. TGA results showed that the addition of MWCNT resulted a substantial increase of the thermal stability and char yields of the nanocomposites compared to the neat PUIMs. TEM experiments show morphology and good dispersion of nanotubes in polymeric matrix. The X-ray pattern of the PUIMs/MWCNT nanocomposites (8a) showed the combined peaks appearing in the MWCNT and pure PUIMs (7).