In this study, the optimisation structure and the UV-Vis spectra of a chlorophyll-b molecule are observed underground state condition using the density functional theory and the time dependent density functional theory.The optimisation of a chlorophyll-b molecule is done using several basis sets and it is found that the cc-pVDZ basis set is the most suitable basis set since it gives the most minimum energy. There are two structures of chlorophyll-b constructed; one is having phytil chain perpendicular with the main group while the other one is having phytil chain parallel with the main group. The two structures in chlorophyll are given similar results; the difference only at the direction of curvature at chain. This happens because the optimised structure must calculate whole system and the SCF must be achieved. Furthermore, as UV-Vis absorbance spectra is calculated, the two maximum peaks are observed at 448 nm and 590 nm.