The use of seaweed has other benefits that do not require extensive breeding ground sufficiently in marine waters, the oil content of 30-50% dry weight and serves as an absorber of carbon from the air, thereby reducing the greenhouse effect. The potential of seaweed as the raw material for making biodiesel is also supported by the content of fatty acids contained in seaweed. Biodiesel production from Padina sp. with CaO by using a catalyst named Psammotaea elongata clamshell have been conducted in the Laboratory of Microbiology, University of Cenderawasih. CaO catalyst have made through calcination process at 1000°C for 2 h and characterized by using scanning electron microscopy. Here, we showed that there are two stages, such as the esterification for 2 h and is catalyzed by H2SO4 1.25% with the ratio of oil and methanol 1:6 and transesterification with the ratio of oil and methanol 1:12 with 1% CaO catalyst at 65°C for 5 h, then biodiesel are characterized viscosity 2.0 cSt, density: 0,66 g/ml, acid number: 1.25, saponification: 106.59 mg KOH/g and power fuel: 45-50 sec. Padina sp seaweed oil can be converted to CaO catalyst from Psammotaea elongata clamshell into biodiesel with the percentage of biodiesel produced from one gram of oil amounted to 5.76%.