i am looking for a free tool to Create ER diagrams for my DB.
db2 data studio is not able to save images when i select 15-20 tables.

any s/w ?

You can try E/R Studio from Embarcadero:
http://www.embarcadero.com/products/erstudio/

ER/Studio is not free (about $4500.00) but you can get a free 14 day trial license.

An ER tool that only costs about $150.00 and supports DB2 is Happy Fish (http://www.polderij****/happyfish/). You can get a free trial for this product also.