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Highlights • Pn waveforms are sensitive to the gradients of crust-mantle transition zone (CMTZ). • Southern Korea features sharp Moho while eastern China Moho shows variability. Abstract The seismic velocity structure of the crust-mantle transition zone (CMTZ) provides key constraints on crustal dynamics, and can be studied with Pn wave which propagates horizontally along the Moho discontinuity. In this study, we first explore an effect of velocity-gradient structures in the CMTZ through synthetic Pn waveforms, and demonstrate that the shape of Pn changes from step-like to pulse-like when the CMTZ becomes from sharp to a gradient velocity structure, respectively. We then use Pn waveforms of the regional earthquakes (the July 2012 Mw 4.8 earthquake in east China and the September 2016 Mw 5.1 earthquake in South Korea) to examine the CMTZ structures to the west and east of the Yellow Sea and thus to constrain tectonic affinity between the eastern China and southern Korean Peninsula. The Pn waveforms from the Mw 4.8 event in east China show that the Moho in southern Yangtze Craton is sharp whereas there may be a gradient structure in CMTZ with a 6–10 km thickness in northern Yangtze Craton. The Pn waveforms from the Mw 5.1 event in South Korea show that the CMTZ in South Korea is also sharp, similar to the southern Yangtze Craton. Observed difference in CMTZ structures suggest that Sulu Orogenic Belt may extend along the north of the Gyeonggi Massif in Korea. [ABSTRACT FROM AUTHOR]