FtsZ is a tubulin homolog essential for prokaryotic cell division. In living bacteria, FtsZ forms a ringlike structure (Z-ring) at the cell midpoint. Cell division coincides with a gradual contraction of the Z-ring, although the detailed molecular structure of the Z-ring is unknown. To reveal the structural properties of FtsZ, an understanding of FtsZ filament and bundle formation is needed. We develop a kinetic model that describes the polymerization and bundling mechanism of FtsZ filaments. The model reveals the energetics of the FtsZ filament formation and the bundling energy between filaments. A weak lateral interaction between filaments is predicted by the model. The model is able to fit the in vitro polymerization kinetics data of another researcher, and explains the cooperativity observed in FtsZ kinetics and the critical concentration in different buffer media. The developed model is also applicable for understanding the kinetics and energetics of other bundling biopolymer filaments.