Abstract:Variation in mating system and genetic diversity of seed samples collected 3 years before and 3 years after thinning were examined in a clonal seed orchard of Pinus tabulaeformis Carr. in Liaoning province. Allozyme markers with 10 loci were used in the study of genetic diversity. The multilocus outcrossing, selfing and inbreeding rates of the seed orchard before thinning in 1993 were 0.962, 0.038 and 0.141 respectively. The multilocus outcrossing rate in the thinned seed orchard in 1996 and 2000 decreased to 0.795 and 0.801 respectively. At the same time the multilocus selfing rate increased to 0.205 and 0.199 and the genetic diversity indices A and P decreased to different degrees after thinning. The results showed that thinning based on phenotype had a strong influence on mating system but a lesser effect in genetic diversity indices. Factors influencing the mating system and genetic diversity are also discussed.