Historic buildings have significant cultural, scientific, and aesthetic value, and show the accomplishments of past eras. Unfortunately, many of wooden historic buildings are destroyed by fire disasters. The reaction-to-fire performance of timber was changed after several hundred years of natural weathering. In this study, the effect of weathering on the reaction-to-fire performance of untreated and fire retardant treated timber are investigated. The conventionally treated cedars were weathered in accordance with a standard construction material durability test method (JSTM J7001), three accelerated weathering test methods (J60, J20 and J4), and a standard weathering method (NT FIRE 053 Method A), respectively. Then, the effect of weathering on phosphoric acid and paraffin treated timber samples was examined by comparing the reaction-to-fire performance of chemically treated timber before and after weathering. The results show that the use of fire retardant can reduce the ignition time, the peak heat release rate (pHRR) and total heat release (THR). The weathering process increases both the pHRR and THR, but reduces the ignition time, which infer a relationship between the two processes of sufficient pyrolysis.