Abstract: Characteristics of Unintentional Modulation On Pulse (UMOP) are key and fundamental for radar Specific Emitter Identification (SEI). In this paper, the common used unintentional frequency and phase modulation features are compared with the consideration of Doppler effect. Under the assumptions of Gaussian White Noise (GWN) and moderate to high Signal-to-Noise Ratio (SNR) level, a simplified observation model of the intrapulse phase is presented. Feature definition is carried out, based on the analysis of existing feature extraction procedure. Then the separabilities of features are theoretically compared by using a criterion derived from intra-class and inter-class scatter matrixes. Result shows that the unintentional phase modulation on pulse feature is more robust to noise. This result is validated by simulations and real SEI tests with 104 radar instances.