Abstract: To evaluate the effect of adding an inoculant (LAB) or/and molasses (M) on the fermentation quality of mixed silage of hull-lessbarley straw and perennial ryegrass during ensiling,an experiment was designed with the following treatments: control,LAB,molasses and LAB+molasses. Six silos per treatment were opened 7,24 and 45 days after ensiling and the fermentation quality was measured. Adding LAB accelerated the onset of lactic acid fermentation; however,there were increases in butyric acid and ammonia/total N during the latter stage of ensiling due to the lack of fermentable substrate. The addition of molasses supplied the fermentative substrates,which improved the fermentation quality and significantly (P<0.05) increased the lactic acid content,significantly (P<0.05) reduced pH and decreased propionic and butyric acid contents. Combination addition of LAB and molasses significantly improved the fermentation quality of mixed silage compared with LAB addition alone,but there were no significant differences in fermentation quality between silages treated by M alone and by M plus LAB. It is concluded that the lack of fermentation substrate is the major limiting factor for mixed silage of hull-lessbarley straw and perennial ryegrass.