Anaerobic co-digestion of olive mill wastewater (OMW) with other natural Substrates containing high level of nitrogen source such as olive mill solid waste (OMSW) is a new approach for upgrading the anaerobic digestion of OMW. This study is the first investigation that has optimised the amount of OMSW that should be used as a co-substrate to compensate the lack in OMW without the need to add chemical nitrogen Substances which are not environmentally and economically desirable. The experiments were carried out on laboratory-scale batch digesters operated at a mesophilic temperature (37 degrees C). Each digester was fed with OMW as a main substrate and OMSW as a co-substrate at different amounts of 28, 56, 112 and 150 g TS/L.OMW respectively. The results indicated that the optimal rate of OMSW used as co-substrate was about 56 g TS/L.OMW. Moreover, it was shown that co-digesting OMSW with OMW in ail optimal ratio has increased biogas production and COD removal efficiency from 11.17+/-2.5 to 30.5+/-2.5 L/L.digester and from 44.5+/-3 to 83.4+/-2%, respectively. Furthermore, it has reduced the start-up time of steady biogas production from 65+/-25 to 28+/-15 days.