AbstractA rosin sizing agent designed to impregnate wood and immobilize copper in wood cells for protection against decay was investigated. Poplar (Populus ussuriensis) wood was impregnated with combinations of 3% CuSO4 solution and 1%, 2%, or 4% rosing sizing agent. The decay resistance of treated wood blocks was measured by a soil-block culture method. After a 12-week decay test, the weight losses of untreated control blocks were 70.45% by Trametes versicolor and 61.84% by Gloeophyllum trabeum. The wood decay resistance was also slightly improved by the treatment with only the rosin sizing agent. However, after being treated with the rosin sizing agent and CuSO4, the wood had great decay resistance. The average weight losses of the samples degraded by fungi were less than 4%. Notably, the leached wood blocks had a weight loss of less than 3%. After leaching, the copper content in the leachates was analyzed by atomic absorption spectroscopy (AAS). Results showed that the amount of copper ions released from the samples treated with the copper-rosin solutions was half those from the samples treated with copper alone. Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX) proved that the copper element was still in the cell lumens of leached wood blocks, which is consistent with the results of AAS analysis. This signifies that the rosin sizing agent is very helpful to fix the copper preservative in wood.