Dynamic parameters of physiology including embryo ratio (ER), embryo length to endosperm length, dry weight ratio between embryo and endosperm (DWR) and respiration rate (RR) in American seng (Panax quinquefolium L.) seed were investigated. According to the changes of ER during seed stratification, the duration of embryo afterripening could be divided into three stages as ① embryo slow growth stage (ESGS), ER increased from 7.31% to 20.48% (0.16% day^1); ② embryo rapid growth stage (ERGS), ER increased to 80.98% (0.61% day^1) (ESGS+ERGS=morphological afterripening stage (MAS)) and ③ physiological afterripening stage (PAS), ER increased to 88.50% (0.094 day^1) only. DWR increased slowly from 0.20% to 2.76% (0.016% day^-1) in MAS and rapidly to 8.81% (0.061% day^1) in PAS. The RR correlated significantly with ER as well (r=0.8934>Γ_0.01 0.6610). The steep increment of both DWR and RR in PAS indicated that the PAS was not a static stage although the ER was not changed too much. All of these may provide some information for understanding the dormancy mechanisms of American seng seed.