In this paper, for the first time, we report on systematic theoretical and experimental investigation of Phase Contrast Optical Tweezers (PCOT) which could be an indispensable tool for micromanipulation of the transparent micro and nano objects such as biological tissues and vesicles. The quadrant photodiode detection scheme and the power-spectrum calibration method is shown to be valid for this case. We have shown that the phase objective with new designed phase plates can provide nearly aberration-free condition at a desired depth. This could be a valuable advantage for simultaneous in-depth micro-manipulations and visualization of the sample.