To investigate the relation between organic matter disappearance (OMD) and acetyl bromide lignin disappearance (ABLD) of tropical legumes, an experiment measuring OMD and ABLD of Siratro (Macroptilium atropurpureum) and Greenleaf desmodium (Desmodium intortum) was conducted. Materials were leaf and stem of Siratro at 23 and 63 regrowth days and of Greenleaf desmodium at 35 and 63 regrowth days, respectively. Using in vitr incubation with rumen fluid and pepsin, OMD and ABLD were measured. Rumen fluid was collected from three goats fitted with rurnen fistulae and fed a hay. Using Morrison’s method ABL was determined. The results obtained were as follows: In each material ABL content was higher than acid detergent lignin (ADL) content. Less value of ABL/ADL was observed in Siratro comparing with Greenleaf desmodium. Though the result showed higher OMD and lower ABLD of Siratro than OMD and ABLD of Greenleaf desmodium and suggested a little different contribution of ABLD to OMD between two legumes, there was positive correlation (p<0.05) between OMD and ABLD in each legume. Authors suggest that ABL is available index for investigating the relation between OMD and lignin of tropical legumes.