A continuum model of the configurations of multiwalled carbon nanotubes (MWNT's) is studied in this paper. The conformation energy of the MWNT is proposed, and the shape equations for a MWNT are derived. From the shape equations, the solutions corresponding to the straight carbon nanotube and coiled carbon nanotube are given respectively. The configurations of the ring MWNT and the plane curve MWNT are also determined theoretically. It is revealed that there are only a very few valid values of the ring radius of the ring MWNT in the case of the fixed parameters of its cross section. The mismatched effect between the interlayer lattices enriches the variation of ring MWNT's with different size. It is found that the conformation energy per unit arc length of the ring is double of that of straight MWNT for same tube cross section. Typical configurations of the plane curve MWNT obtained from the shape equations agree well with the TEM images of the nanotubes. The total conformation energy of the plane curve MWNT is in the same order of the ring MWNT one for enough long arc length.