  The Cryo-EM 3D particle reconstruction is essential for identifying protein
and uncover the biological mechanism of the macro-molecules. In this paper, we
use Kam method for reconstruction. Kam method is \textit{ab-initio}, and it
assumes that the projection angles of the particle are uniformly distributed.
Based on the data covariance matrix, we compute the radial frequency component
of the matrix. The particle density function can be obtained by the radial
frequency component and the angular frequency basis function. In order to
uniquely and accurately identify the radial frequency component, an initial
guess of the structure is applied. Experiment shows that Kam method works for
low resolution particle reconstruction. Further improvement can be made by
including projection angle distribution in covariance matrix, and applying the
fast algorithm to enhance computation speed.
