To prepare a gel-forming poly-L-guluronic acid (Poly-G) from no guluronate-rich Laminaria japonica, a new hydrolysis method was employed with a lower HCl concentration (0.025-0.15 M) and a shorter treatment time (5 min). The Poly-Gs were set to measure purity, presence of poly-L-guluronic block, molecular weight distribution, polymer yield, viscosity, and compressive gel strength. Finally, the Poly-G was tested to embolize the renal vascular system by using a rabbit model and angiography. Optimized Poly-G could be selected with respect to wt% concentration, polymer yield, gel-forming stability, viscosity, and gel strength as an endovascular embolizing agent. Overall, 0.4-0.6% of 0.03 M-Poly-G obtained from acid treatment with 0.03 M of HCl had molecular weights greater than 80 kDa, and the best gelling capacity with an injectable viscosity (30-120 cP). It was successfully delivered into the vascular bed of a rabbit kidney and was shown angiographically to embolize the renal vascular system.