Emperors and members of prominent clans were at first buried in traditional-style kofun well into the 7th century, following the practices of the earlier Kofun period.
Inside of the Ishibutai ancient tomb (Daily Yomiuri photo)
Along with Buddhism however, the practice of cremation was introduced. In time the custom of cremating bodily remains came to be adopted by all, from the Emperor, the nobility and downward to the commoners.
It is likely that as resources were diverted from the building of large tomb mounds (kofun) to temple building, cremation made more sense as resources could be conserved. The earliest written record of cremation in Japan refers to the priest Dosho (d. 700. or the 4th year of the reign of Emperor Monmu). Certain kofun exhibit a transitional character. For example, the Nakaoyama Kofun (Asuka-mura, Hirata) , which was designed with an octagonal mound and a minutely worked stone crypt for the deposition of cremated bones.
Heritage of Japan 
First of all, I really appreciate what you did with this webpage, it helped me in my studies of japanese history a lot.
I have a question for you, during Kofun era only nobles and political leaders were entombed if im correct? So, I was wondering, how were the ordinary people burried? Were they cremated, or maybe even burried in pits?
I have the answer somewhere, will get back to you later.