For a vicinal Si(001) 2×1 surface, a row of extra spots has been clearly observed along the reciprocal lattice rod in energy-filtered reflection high-energy electron diffraction (ER-RHEED) pattern. The glancing angle dependence of these extra spots along (0 0), (1/2 0) and (1 0) rods was simulated by kinematical analysis based on a monatomic regular step surface. Behavior of these extra spots was reproduced well by the calculations and the mean terrace length along the [110] direction was estimated. It has been found that this method can evaluate very small vicinal angle near 0.1^o, which is superior to low energy electron diffraction (LEED) method in this point.