Cementation of hazardous liquid waste is one of the methods to minimize its detrimental effect on the environmental quality and human health This research purpose was to study the effect of natural fiber composition and temperature on quality of the cement block reinforced by coconut (Cocos nucifera) fiber and bamboo (Bambusa vulgaris) fiber. This research was pursued by adsorbing stronsium waste and krom using zeolite. Thirteen percent volume of zeolite was mixed with 0,3 of water/cement ratio. Composition of natural fiber was varied by O,OOv/o, 0,05v/o, 0,1 Ov/o, O,25v/o, 0,50v/o, 0,75v/o and 1,00v/o. The cement blocks produced were heated at OOC,50°C, 100°C, 150°C, 200°C and 250°C for 10 minutes and then determined their compressive strength and leaching rate. The optimum composition of natural fiber causing increasing of mechanical strength has been founded at 0.50% vlo of fiber On that composition the axial force resistance of fiber is higher than the radial one. The hydration reaction completely works when cement block is heated until certain temperature that results in the increasinJ! of its compressive strength. However, the compressive strength of cement block heated up to 250 C is still beyond the standard. Based on its compressive strength, the bamboo (Bambusa vulgaris) fiber is more feasible than coconut (Cocos nucifera) fiber for reinforcing cement block. Heating just influences on the physics properties of cement block But, the ability of block cement to immobilize a matter is affected by properties of matters. Susetyo Haria Putero, Nunung Prabaningrum, Widya Rosita Jurusan Teknik Fisika, Fakultas Teknik, Universitas Gadjah Mada