The physical properties of cement-fly ash grouts containing organic fiber and superplasticizer (SP) for isolation of low-level radioactive wastes are presented. In order to produce a workable, crack-resisting, impermeable, and durable grout, the grout mixes studied contain cement, fly ash, water, polypropylene (PP) fiber, and SP. Laboratory studies included viscosity, bleeding, setting time, compressive and flexural strengths, pore structure, water permeability, and durability of grout mixes. The findings indicated that grouts containing organic fiber were more crack-resisting and less vulnerable to environmental changes, but showed higher viscosity and permeability. With the incorporation of SP in grout mixes, the adverse effects introduced by organic fiber were corrected with additional improvements in viscosity, flexural strength, water permeability, and durability against wet-dry cycling and sulfate attack.