In order to improve the mechanical properties and decay resistance of plantation grown poplars, the cathay poplar (Populus cathayana Rehd.) wood was treated with the montmorillonite (MMT)/didecyl dimethyl ammonium chloride (DDAC) compound preservative, using a full-cell process. The percentage of weight gain, retention of DDAC, distribution of organic montmorillonite (OMMT) in wood, compression strength perpendicular to the grain and decay resistance were determined. The results show that the percentage of weight gain increased with the introduction of OMMT, but the retention of DDAC decreased. Most of OMMT clusters infiltrating into the wood settled in wood cell lumen. A few small clusters and exfoliated layers passed through the pits, but could hardly infiltrate cell walls. Compared with wood samples treated with distilled water, the compression strength perpendicular to the grain of the samples treated with the compound preservative increased by 27%. The decay resistance of compound preservative treated samples also increased compared to those of DDAC treated samples.