This paper presents the linearity analysis of Phase-Domain Delta-Sigma Modulator (PDDSM). This study shows that, besides the reference phases in the DSM feedback path, there are additional parameters which have significant impact on the PDDSM linearity, namely the quantizer sampling frequency and the input driving frequency. Comprehensive analysis of their impact on PDDSM linearity has been performed and validated with behavioral model simulation in MATLAB with well-matched results. This analysis provided additional theoretical understanding on the PDDSM linearity and the design guideline to achieve high linearity PDDSM.