Poly(amide-co-imide) (PAI)/multi-walled carbon nanotube (MWCNTs) composites were prepared by using solution mixing with ultrasonication excitation in order to investigate effects of MWCNTs on rheological properties and thermal curing behavior. Steady shear viscosity of the composite showed bell shaped curves with three characteristic patterns: shear thickening, shear thinning, and Newtonian plateau behavior. Both storage modulus and complex viscosity were increased due to higher molecular interaction than that of the pure PAI resin. Especially, hydrogen peroxide treated MWCNT/PAI composites had the highest storage modulus and complex viscosity. Glass transition temperature of the PAI/MWCNT composite was increased with increasing MWCNT content and thermal curing time since the mobility of PAI molecules was reduced as more constraints were generated in PAI molecular chains. It was found that thermal curing conditions of PAI/MWCNT composites are determined by considering effects of weight fraction and surface modification of MWCNTs on internal structure and thermal properties.