Structural reliability analysis was carried out on the Mansonia altissima timber, to ascertain its structural performance in timber roof truss system. Structural analysis of the timber truss was in accordance with Eurocode 5 (2004) and was carried out under the Ultimate Limit State of loading. A developed MATLAB (2010) programme was employed for reliability analysis of the timber roof truss of Mansonia altissima timber so designed, to ascertain its level of safety using GA-based First-Order Reliability Method. The uncertainties in the strength and load variables were accommodated in the reliability analysis. The result of the analysis revealed that the Joint failure mode is the critical safety index that is minimum safety index among the failure modes of the truss under the design conditions. The Mansonia altissima timber was found to be a satisfactory structural element for timber roof truss at depth of 75mm, breadth of 50mm and under the ultimate limit state of loading with the corresponding of 2.58. Sensitivity analysis proves that the degree of reliability of the timber roof truss can be improved if crosssections of species, diameter of nail at joint, pitch of truss and loadings are suitable selected.