Most plantation forests are even-aged monocultures, which are supposed to have low ecological stability, resource use efficiency and biodiversity. So, in recent years, forest managers interest in plantations with mixed species composition. Although it is expected that various ecosystem functions of a forest will be enhanced by the mixture, evidences showing the positive effect are limited. The objective of this study is, thus, to clarify the effects of mixture of broadleaved species on multiple ecosystem functions of a coniferous plantation forest in northern Japan. The study was conducted in a 34-year-old Picea glehnii plantation (2.65ha). We examined mixed stands with naturally regenerated Betula ermanii and Quercus crispula, in comparison with pure P. glehnii stand. The biomass of overstory trees was not statistically different among the stands. In contrast, in the mixed stands, the biomass of understory vegetation was significantly greater. The indices representing plant and insect species diversity were similar among the stands.As seen above, the effect of mixture with broadleaved species was limited; major differences were only found in understory vegetation. However, with considering abundant natural regeneration, which could increase some functions in the future, in the mixed stands, the effects should be reevaluated in a later stage of stand development.