buying a condo

Yes in a typical Thai building where all the units are a small room plus bathroom totaling 26sqm or less. In most cases the entire bathroom floor will get wet when you take a shower and if you want any type of kitchen you will need to install on your balcony if you have one.

thanks for advice.which area of bangkok can i find it?
can i own it or its just a 30 years leasing for foreigners?

They are all over the city but not usually next to skytrain or subway.  Look for low rise buildings (under 10 floors) that have only a small balcony and small window for bathroom as seen from street for each unit.  Others have a full width balcony and bathroom is located on entry side of unit.  All generally 26sqm or less.

The small unit buidlings usually have very few foreigners that buy so it should be no problem to purchase.  Foreigners fall under a quota which means they can only own up to 49 percent of the total square meters of a building. The building juristic person will have foreign ownership data.

thanks for ur help

its a little complicated for me,i saw an advertisement in internet,at the link below:
http://phuketbuyhouse.com/2437/resident … 299955-thb
the price of the leasing of this house which is mentiond in the lind is 300000baht,i wanna know its 300000baht for 30years or pear year???hence its said its for sale

The advertisement leaves a person very confused so I suggest you call individual listed for that property.  Perhaps it was meant to be confusing so you are forced to call.