Epoxy-modified bismaleimide (BMI) foams were prepared through prepolymerization and foaming. Density-adjustable BMI foams with closed-cell structures were produced. The influence of prepolymerization time on bubble growth and morphology, as well as the compressive and heat resistant properties of the foams, were thoroughly investigated. BMI prepolymerization was an effective method for controlling the bubble growth rate, the bubble size and its distribution, and the foam density. As the prepolymerization time increased, the matrix viscosity, the number of cells per unit volume and the foam density increased, whereas, the bubble growth rate and cell size decreased, and the cell size distribution became narrower. The compressive strength and modulus increased with increasing prepolymerization time. The dimensional stability temperatures of the BMI foams were all above 200 degrees C, as determined by their base materials, and were unaffected by prepolymerization time.