Hi Radhakrishnan
Although an EPUB is essentially a zipped folder of web code there are a couple of files in there that are not to be zipped when packaging into EPUB. So really you should get into the habit of using a utility. There are a few free options:
eCanCrusher (as Paul suggests) is available from Rorohiko https://www.docdataflow.com/ecancrusher/ and is cross platform but if you are on a Mac I suggest ePubZipUnzip – https://bit.ly/ePubZipUnzip
Also pagina EPUB-Checker from Pagina will combine packaging with validation using epubcheck – https://www.pagina.gmbh/produkte/epub-checker/
Good Luck!
Ken Jones
Circular Software
http://www.circularsoftware.com