A one-third scale single unit of 3-storey tunnel form building with foundation beam was designed, constructed and tested under out-of-plane lateral cyclic loading. This building was designed according to BS8110 with no provision for seismic loading. Tunnel form building was initially tested at ±0.01% , ±0.1%, ±0.25%, ±0.5%, ±0.75%, ±1.00%, ±1.25% and ±1.5% drifts until the structure lost its lateral strength (strength degradation). Then, the specimen was repaired and retrofitted using steel plate and angle which was positioned at the wall-slab joints that had suffered severe damages. A similar drift of displacement before retrofitting was repeated for the specimen after being retrofitted. Visual observation of damages, hysteresis loops, stiffness, ductility and equivalent viscous damping (EVD) of the specimen were analyzed and compared before and after retrofitting. The proposed retrofitting technique has increased its lateral strength by 26%. Fewer cracks occurred after retrofitting the specimen compared to before retrofitting. It can be concluded that steel plate and angle can strengthen the wall-slab joint and this method can be used to retrofit the tunnel form building if it had been damaged caused by earthquake excitations.