In vitro study was carried out to determine the effect of replacing clover hay with olive cake silage with or without fibrolytic enzymes on rumen fermentation and gas production and dry matter disappearance. The experimental diets consist of 50: 50 concentrate: roughage ratio (on DM basis); the experiments divided into four groups, the first group diet was 50% concentrate feed mixture (CFM): 50% clover hay (control). The second group diets were replaced clover hay by olive cake by 25, 50 and 75% for T1, T2 and T3, respectively. The third group diets were replaced clover hay by olive cake silage by 25, 50 and 75% for T4, T5 and T6, respectively. The fourth group diets were replaced clover hay by treated olive cake silage with fibrolytic enzymes by 25, 50 and 75% for T7, T8 and T9, respectively. The results of ensiling olive cake showed that treated olive cake silage was recorded the lower values in pH and NH3-N compared with untreated silage. Results showed a significant decrease in gas production and relative gas production to DM and fiber fractions in all treatments compared with control. DM disappearance showed that T1, T4, T7 and T8 recorded a significant increase compared with control. The current findings reflects a potential possibility of replacing clover hay with olive cake silage especially treated with fibrolytic enzymes.