Recent studies have shown that genotypic diversity of a plant can affect ecosystem functions and insect communities as plant species diversity does. However, few studies have explicitly referred to the mechanisms behind the effects of genotypic diversity on ecosystem functions and insect communities, and many of them are limited to herbaceous plants. In this research, we used a woody plant species, Betula ermanii, and investigated how birch genotypic diversity influences its productivity and insect community. Using 6 genotypes, we arranged 64 plots randomly with single-genotype, 3-genotype and 6-genotype plots, and conducted census of plant and insect once per month during growing season. For ecosystem functions, birch genotypic diversity had an additive effect on its productivity, but a positive non-additive effect on leaf chlorophyll concentration. For insect community, birch genotypic diversity had a negative non-additive effect on chewing herbivory and density of chewer, galler and sucker, but an additive effect on miner’s density. In conclusion, in a short-term experiment, plant genotypic diversity can have greatly affected insect community, but have barely affected ecosystem functions.