  Why massless particles, for example photons, can only have two helicity
one-particle states is the main subject of this work. As we know, the little
group which describes massive particle one-particle states' transformations
under the Lorentz transformation is SO(3), while the little group describing
massless states is ISO(2). In this paper, a method which is different from
Inonu-Wigner contraction is used to contract SO(3) group to ISO(2) group. We
use this contraction method to prove that the particle can only have two
helicity one-particle states from the perspective of kinematics, when the
particle mass tends to zero. Our proof is different from the dynamic
explanation in the existing theories.
