Abstract: The arabinogalactans from Larix gmekini and L. olgensis are isolated and characterized, correspondingly, the cotents are 7% and 10.2%, the molecular weights (MW) are 53000 and 44000, respectively. Both species are resemble each other in sugar composition which includes galactose, arabinose and uronic residues, and the ratio of galactose to arabinose is approximately 6. The results of SEM and DTA show that the effect of arabinogalactan on hydration and crystallization of Portland cement is seriously harmful. It is an inhibitor of setting and hardening of Portland cement.