Abstract: Taking 9196 plants of 2-year-old seedlings of Pinus yunnanensis from 140 half-sib progeny planted in the nursery of Southwest Forestry University as research materials, seedling height and ground diameter were used as analysis indexes, the data were extracted by writing a computer program according to family sample sizes, and the family heritability was estimated for each family sample sizet. Based on the analysis of the influence of family sample size on the heritability estimation of P. yunnanensis, the critical sample size of the tested population was determined under the condition of ensuring accuracy and saving cost. Results showed that for the half-sib family of P. yunnanensis, family heritability estimation of seedling height and ground diameter were significantly different among the family sample sizes, there were significant differences between the seedling height(P<0.01) and ground diameter(P<0.05). The family sample sizes had significant effects on the family heritability estimation of seedling height and ground diameter of P. yunnanensis. The estimation accuracy increaseed with the increase of the family sample size. When the family sample size of P. yunnanensis was 70, the mean value of the family heritability was relatively stable, while the standard deviation and coefficient of variation were relatively low. When the family sample size of P. yunnanensis was greater than 70, the accuracy of the family heritability estimation value was guaranteed, and the actual production and test requirements were also met.