Abstract: The research on arborous layer biomass and productivity of 5 kinds of Mongolian oak forests （Quercus Mongolia）was performed.The results show that according to the biomass,the Mongolian oak forests are ranked in a descending order as follows：filbert oak forest（249754.45kg/hm²）,high production oak forest（184750.00kg/hm²）,lespedeza oak forest（128312.00kg/hm²）,rhododendron oak forest（124573.16kg/hm²）,oak coppice （42974.50kg/hm²）.The biomass of most stands is distributed in the trees which are bigger in diameter and less in number.The comparison between the biomass of the oak forests and that of other secondary forests in northeast region of China shows that filbert oak forest＞bi rch forest＞high production oak forest＞miscellaneous tree forest＞lespedeza oak forest＞rhododendron oak forest＞mountain polar forest＞hard broad-leaved forest＞oak coppice.The net productivity of the oak forests is 1870 1.3kg/hm²·ɑ for filbert oak forest,7682.6kg/hm²·ɑ for oak coppice,7622 .1kg/hm²·ɑ for rhododendron oak forest,7588.5kg/hm²·ɑ for lespedeza oak forest,6984.0kg/hm²·ɑ for high production oak forest.