Laccase is a kind of polyphenol oxidases and frequently found among higher plants and fungi. We previously cloned a laccase-coding gene from a cDNA library of the Royal Sun medicinal mushroom, Agaricus brasiliensis. However, our efforts to yield the wild-type laccase was unsuccessful probably due to its instability. Therefore, in this study, we tried to improve the stability of the laccase by the ancestral design method. The designed laccase was successfully yielded by a Pichia pastoris secretory expression system and then purified. Thermal stability measurements of the purified laccase showed that the protein is nicely thermostable; its half-life at 70ºC is almost 70 min. The half-inactivation temperature of the protein after heat-treatment for 10 min is higher than 80ºC. The kinetic properties are reasonably similar to those of a laccase from a closely related species, A. bisporus. We also found that the designed laccase can catalyze the polymerization of several monomeric polyphenols.