We evaluated effective time constants of random telegraph noise (RTN) with various operation timings of in-pixel source follower transistors statistically, and discuss the dependency of RTN time constants on the duty ratio (on/off ratio) of MOSFET which is controlled by the gate to source voltage (VGS). Under a general readout operation of CMOS image sensor (CIS), the row selected pixel-source followers (SFs) turn on and not selected pixel-SFs operate at different bias conditions depending on the select switch position; when select switch locate in between the SF driver and column output line, SF drivers nearly turn off. The duty ratio and cyclic period of selected time of SF driver depends on the operation timing determined by the column read out sequence. By changing the duty ratio from 1 to 7.6 × 10-3, time constant ratio of RTN (time to capture 〈(τc〉)/(time to emission τe〉) of a part of MOSFETs increased while RTN amplitudes were almost the same regardless of the duty ratio. In these MOSFETs, 〈τc〉 increased and the majority of 〈τ e〉 decreased and the minority of 〈τe〉 increased by decreasing the duty ratio. The same tendencies of behaviors of 〈τc〉 and 〈τe〉 were obtained when VGS was decreased. This indicates that the effective 〈τc〉 and 〈τe〉 converge to those under off state as duty ratio decreases. These results are important for the noise reduction, detection and analysis of in pixel-SF with RTN.