Abstract: In order to study effect of molecular structure on three-photon absorption(3PA) characteristics, two novel fluorene-based derivatives: 2, 7-bis(2-(4-methoxyphenyl)ethynyl)-9,9-dioctyl-9H-fluorene(A) and 2-bromo-7-(2-(4-methoxyphenyl)ethynyl)-9,9-dioctyl-9H-fluorene(B) were synthesized. The 3PA cross-sections of both the compounds were tested by using a Q-switched Nd:YAG laser at 1064nm. The 3PA cross-sections were obtained, i.e., (6.03±0.6)×10-76 cm6·s2/photon2 and (4.25±0.4)×10-76cm6·s2/photon2respectively. The geometries in the ground states and electronic structures of both the compounds were investigated by density functional theory methods and time-dependent density functional theory methods of Gaussian 03, and the impact on the three-photon absorption characteristics was analyzed.