  We present a new method for determining the age of late-K type pre-main
sequence (PMS) stars by deriving their surface gravity from high-resolution
I-band spectroscopy. Since PMS stars contract as they evolve, age can be
estimated from surface gravity. We used the equivalent width ratio (EWR) of
nearby absorption lines to create a surface gravity diagnostic of PMS stars
that is free of uncertainties due to veiling. The ratios of Fe (818.67nm and
820.49nm) and Na (818.33nm and 819.48nm) absorption lines were calculated for
giants, main-sequence stars, and weak-line T Tauri stars. Effective
temperatures were nearly equal across the sample. The Fe to Na EWR (Fe/Na)
decreases significantly with increasing surface gravity, denoting that Fe/Na is
a desirable diagnostic for deriving the surface gravity of pre-main sequence
stars. The surface gravity of PMS stars with 0.8 solar mass is able to be
determined with an accuracy of 0.1-0.2, which conducts the age of PMS stars
within a factor of 1.5, in average.
