This study was carried out to investigate the effects of IBA(indole-bufrric acid) on the rooting and root growth of air-layered part of Pinus thunbergii for miniature tree material. The experiment was performed on April 13 and lune 11, 2000 in the humidity controllable greenhouse condition. In April experiment, IBA treatment with 5000ppm at girdling part shortened about 20 days for rooting than those of control and 1000ppm treatment. IBA treatment in June did not affect on the period required for rooting, but the rooting of all the air-layered seedlings was faster than that of in April. On the other hand, IBA treatment in April did not affect on the number of root grown at girdling part and the root biomass, but the treatment in June increased the number of root and the root biomass of air-layered Pinus thunbergii.