Femtosecond laser nano-ablation of Ti by short-pulse laser irradiation (800 nm/40 fs) is studied in the laser fluence range of 0.07-0.5 J/cm². To determine the ablation threshold, the ablation rate dependence on laser fluence is precisely measured. Multi-shot ablation threshold of titanium is found to be 0.074 J/cm² in which value is good agreement with that reported previously by other group. To discuss the ablation mechanism, all the ablation thresholds for metals previously published are plotted as a function of work function and melting temperature. We found that the ablation thresholds correlated with work function of metals. Experimental data suggested that the femtosecond laser ablation is mainly due to multi-photon absorption and optical field ionization.