Among various types of fiber reinforced polymer (FRP) materials, sheet-formed materials are commonly used for the strengthening of reinforced concrete structures. Basalt fiber sheet has shown a good durability to a harsh environmental condition comparing to other typical fiber sheets, such as glass and carbon fiber sheets . This study investigates the debonding failure characteristics of the basalt fiber sheet for its application in the field. Fundamental mechanical characteristics were evaluated prior to the bond fracture test. Experimental variables include the number of fiber sheet layers, bonding length and bonding width. Debonding failure mode was typical at the bonded interface and the failure strength increased as the number of layer increased. From the results, the effective bond length and effective bond strength were calculated and compared with the literatures. It was also found that the debonding failure strength was a function of the bonding width rather than the bonding length. Based on the test data, an equation to calculate the effective bonding length and strength was suggested.