Abstract: With the minirhizotron technique, the new root dynamics of 19 year old loblolly pine plantation was studied in Louisiana of US. The results are as follows: the loblolly pine root count density increases in the growing season and reaches its climax in January or February, the end of the growing season. The root count density of the trees, on which drought has been imposed by excluding the throughfall, decreases from the depth of 9.3 cm with the soil depth, and that of the trees without imposed drought is considerable higher even though in the soil depth of 80 100 cm, ranging from 3.81 to 5.12 root count·m-1, and more than 10 times of that of those trees with imposed drought. The new roots of loblolly pine begin to initiate in March, and rapidly increase until June, when it reaches its climax, it begins to decrease slowly. The new root increment from May through July accounts for 41.8% of the total. Most of the new roots initiate in the soil depth of 0 30 cm. The 11 month long imposed drought significantly decreases the root count density and new root initiation of the loblolly pine, with the mean root count density and new root count being 3.27 root count·m-1 and 156 counts, respectively, for the trees without imposed drought, and 1.1 root count·m-1 and 57 counts, respectively, for the trees with imposed drought. The imposed drought also delays new root's initiation by 1 to 1.5 months. The fertilization applied 5 years ago seems not to have effects on new root initiation and growth.