We have obtained the population of the highly excited Rydberg states of sodium atom by two-step excitation using tunable lasers. The field- ionization thresholds for excitation states were measured by using a pulse electric field, which applied to interaction region after the laser excitation. Lifetimes for ns (n=20～24) and nd (n=19～23) states were measured by delaying field-ionization, and compared with the calculation values. The influence of some factors on lifetime is discussed.