The scan PUF, which is based-on the power-up states of scan flip-flops, had been proposed to overcome security issues of semiconductor ICs. IC identification, one of those security issues, requires decent uniqueness along with reliability and randomness. This paper presents two efficient PUF elements' selection methods for scan PUF: uniqueunanimous selection method and unique-majority selection method. These methods classify the scan cells according to their trend of power-up states and prioritize them to extract PUF elements. For experiments, enrollment and validation is performed on 15 chips, which are fabricated with 65nm CMOS process. A statistical analysis on experiments verifies the performance of proposed selection methods.