Abstract: Spatial distribution of galls induced by longan leaf-gall midge(Asphondylia sp.) on Dimocarpus longan Lour.was studied.Data were analyzed by means of 6 aggregation indices,Iwao equation and Talylor's power-law.In addition,the sampling and factors considered are also discussed in this paper.The results showed that(1) The galls mainly distributed in primary lateral veins or midveins on the lower leaf surface;frequency of the gall appearances was similar on either side of the midveins;the gall distribution ratio was highest in the middle section,followed by the upper and the lower section on a longan leaf with the galls on midveins mainly located at the upper parts,while on lateral veins the middle section.(2) All indices indicated that the gall spatial distribution was an aggregated pattern;the basic component was the individual colonies,which mutually attracted one another with an aggregation intensity increasing with density.(3) The aggregation was caused by the environmental factors when m4.01,and by the gall's behavior as well as the environmental heterogeneity when m4.01.(4) The gall density was not significantly affected by the facing direction on a tree.It was significantly higher on the middle and lower parts of a tree than the upper part.(5) On the basis of the spatial pattern analysis and Iwao's m*-m regression formulation,the optimal sampling model was established to be N=t2/D2(3.688 4/m+0.197 2),and the theoretical optimum sampling numbers for different gall densities within the allowable error were calculated.