Do I need transit visa during my flight from korea to kathmandu ?

It depends on the Passport you hold. There is a 72-hour Visa-free Transit for 51 countries. And it can only be used for the appointed cities in China. You can check the details on the visa page of this website. If you have only one stop in China do not including Hong Kong, Macau and Taiwan, and only stay within 72 hours, you can go without visa.