The influence of moisture absorption on the primary (glass) transition (T-a or T-g) and the low temperature relaxations of semiaromatic amorphous polyamides synthesized by isomeric aliphatic diamine and metha or para oriented phthalicdiacids has been investigated by means of differential scanning calorimeter (DSC) and dynamic mechanical thermal analyser (DMTA). The glass transition of semiaromatic polyamides was lowered due to the water absorption, and the beta and the gamma relaxations were as well. From the observed T-g and the difference in the heat capacity, the calculated T-g depression per 1 wt % water content was 12.3 K and the result was in good agreement with the experimental data. The depression of the glass transition may be expressed by the same manner as the plasticization of nylon 6 by water. The depressed beta relaxation observed in the specimen containing a few percent of moisture was splitted into two transitions due to the reduction of water content, of which one was the elevation of the T-beta and another was the simultaneous appearance of the T-gamma, and then the single T-gamma solely was observed for the completely dried specimen. The T-gamma seemed to be merged into or not to be observed by the large and broad T-beta transition when the sample was governed by a few percent of water, then it was emerged from the T-beta due to water desorption. Thus, the T-beta is believed to arise from the intermolecular hydrogen bonding between water molecules or between water and amide groups in wet polyamides. In addition, the gamma relaxation originated from the peptide groups is attributable to the inter- and intramolecular hydrogen bonding between amide groups.