Abstract: Dynamic impedances of pile groups have been studied almost on a same assumption that rigid caps and soil under it do not touch each other in the current literature.This paper account for the dynamic impedance of pile groups without the aboved-mentioned assumption. So, that the soil under caps affects the caps and piles is discussed.In the paper,first, dynamic impedance of the single pile and the single rigid foundation are derived in homogeneous elastic media. Then, the pile-to-soil, soil-to-pile and soil-to-soil dynamic interaction factors are defined and worked out. And a simple method for calculating vertical dynamic impedance of rigidly-capped pile groups in homogeneous elastic media is developed.