The length, width, thickness and weight of the current year and more than one-year-old needles of both bi-needle-cluster and tri-needle-cluster of slash pine (Pinus elliottii) were measured to expose the differences in terms of leaf shape and specific leaf area (SLA), and leaf area index (LAI) was estimate based on measurement of the plantation in Qianyanzhou Ecological Station. The needle’s cross section of tri-needle cluster was approximately in a standard circle. However, cross section of bi-needle cluster was proved to be an ellipsoid. Diameter in thickness direction (1.580mm for 1-year-old needle and 1.568mm for 2-4-year-old needles) was significantly larger than that in width direction(1.422mm and 1.410mm for 1 year old needle and 2-4-year-old needles, respectively). If the cross section of bi-needle was arbitrarily regarded as a circle by using the average diameter of the two directions, there would be an error less than 3%, while the error will be as much as 2%10% if only one of the two diameters is adopted. The correlation between leaf length and leaf area showed that leaf length was significantly related to leaf area. Varied among needle types, SLA of 1-year-old needles showed larger than that of 2-4-year-old needles. Tri-needle leaves have much bigger specific projected area and specific total area than bi-needle leaves, but smaller in specific surface area. The projected LAI of slash pine plantation in Qianyanzhou Ecological Station was 3.61, and the LAI estimated based on the surrounding-surface area and the half of the total surface area of cylinder were 5.12 and 4.52 respectively. All these estimated values of LAI were larger than the direct measurement by optical canopy analysis system.