Frustules of the cultured diatom (CD) with unique porous architecture and high surface area were used as template for electrodes. Iron oxide coating on cultured diatom (CD-FeO_x) was carried out via hydrolysis and metathesis reaction of FeCl₃, which allowed the uniform coating of the FeO_x onto frustules. The relatively large surface area of CD contributed to double-layer capacitance, while the FeO_x provided pseudocapacitance. Results showed that 51% loading of FeO_x on CD significantly enhanced performance to 70.2 F g⁻¹, which was much higher compared with CD only. Supercapacitors remained functional after 1000 cycles and maintained 90% of the original performance. The CD-FeO_x composite was also successfully used to fabricate flexible supercapacitor electrodes.