Analytical dynamics methods of a super thin shearable and extensible elastic rod with background of a class of a single molecule such as DNA are studied based on the plane cross-section assumption and exact cosserat model, which is an extension of the method for Kirchhoffrod [1-8].Relations between geometrical concepts of a rod such as tangent vector and velocity at a point of center line.extension strain, curvature-twisting vector, angular velocity, etc are discussed.Kinematical equations of a cross section of the rod are given.