Dissertation (MSc Biodiversity Conservation)Sustainability of forests globally is threatened by human activities through deforestation for fuel wood, human settlements, cultivation, mining and forest fires. A study was conducted at Kibele forest of Zanzibar to assess extent of encroachment and forest disturbance, and their effect in forest performance based on specific objectives: (1) to assess types of anthropogenic activities and extent of forest encroachment, (2) to determine effect of forest disturbances on stem density, basal area, standing volume and wood biomass. The study also aimed (3) to determine tree species diversity between disturbed and undisturbed natural forest patches, and (4) to determine the effect of encroachment and associated disturbance on extracted wood biomass. Circular plots design was used, and a total of 104 plots of 8 m radius with inter-plot distance of 280 m were laid systematically along 16 temporal transects at 280 m apart. Trees within the plots were identified and recorded. Data were analyzed into simple descriptive statistics using SAS Program, version 9.1 for windows. Results showed that, high extent of encroachment due to farmlands was estimated 82.9 ha (9.9 %) compared to human settlements that was 0.74 ha (0.01 %). Stocking parameters were highly variable (p<0.05). Higher values of stem density, basal area, standing volume and tree biomass were found at undisturbed forest patches corresponding to 894-1068 stems/ha, 1.9-2.8 m23/ha, 10.2-17.7 m/ha and 5.9-10.3 tons/ha respectively. High value of 0.3-0.4 of tree dominance and low value of 1.1-1.3 of tree diversity was recorded at burnt areas. Tree biomass loss was 14.7 tons in year 2012, permanent farms depleted 99.1 % of the total tree biomass. The findings in the study could be concluded that, variation of values of stocking parameters and indices at forest conditions is the results of human activities.