A duplex printing method in a printing apparatus having a duplex printing function printing incoming print data on both sides of a print sheet includes sequentially converting the incoming print data into images corresponding to respective pages and storing the images of the pages, determining whether there are the images having two side pages to be printed on both sides of a single sheet among the images of the pages, if it is determined that there are the images having the two side pages, forming a dual queue having both odd page image related information and even page image related information corresponding to the two side pages, variably scheduling a printing sequence of the pages to be printed using the dual queue and performing the duplex printing function, and repeating the above operations until all of the incoming print data are completely received and printed. Therefore, a memory can be efficiently used, and the scheduling of the printing sequence of the pages to be printed can be simplified.