Abstract: The rule of diurnal variation of net photosynthetic rate of the fine half-sib families of slash pine were studied by Licor-6400 portable photosynthesis system in order to adjust planting measurement. The results indicated that the diurnal variation of the photosynthetic rate (Pn)of the half-sib families of slash pine exhibited double-range cur in typital days of March to August in a year. Others were single-range cur such as January, February, September, October, November, December. The photosynthetic rate(Pn) of one-year old needles is obviously higher than two-year old needles. The photosynthetic rate (Pn) is the lowest in the evenfall of a day. The photosynthetic rate(Pn)of fine half-sib families of slash pine is obviously higher than the general. It is indicated that the photosynthetic ability of slash pine is improved by genetic improvement. Thus the yield and the growth increment are improved of the fine families of slash pine.