F-actin is a semi-flexible macromolecule. Above a few tenths of a percent in volume fraction, F-actin solution undergoes an isotropic (I) to nematic (N) phase transition. By tracking fluorescently labeled F-actin in a network of unlabeled filaments, we studied the diffusion behavior of F-actin across the I-N phase transition and found an abnormal slowdown of longitudinal diffusion after the system enters nematic phase. In contrast, for an ordinary liquid crystalline I-N phase transition, there is an abrupt increase of longitudinal diffusion coefficient at the transition point. We attribute this counter-intuitive phenomenon to counter ion condensation induced reduced repulsion or possible weak attraction between filaments.