Internal crystalline structure of cementite catalyst particles located inside the base of multiwall carbon nanotubes was studied using nanoprobe convergent-beam electron diffraction. The catalyst particles are single crystalline but exhibit combinations of small-angle (∼1°-3°) rotations, twists, and bends along their axial length between adjacent locations. Distortions are most severe away from the base up into the nanotube where the number of walls is large. Near the attachment point to the substrate, however, where few, if any graphene walls exist, particles remain undistorted. This suggests that the stresses generated by the surrounding nanotube distort the catalyst particle during growth.