i wanted to change the site's name and will just add a word on it.
will there be an effect to my pr?
You mean get a new domain name or just change the title?
If you change the domain name, then there will ofcourse be an effect on your PR. You should do a 301 redirect to your new domain from the old one to pass on the link juice to your new domain and have the same PR.
If you are just changing the title of your site, then this will have no effect on your PR.
I'm assuming you're adding a word to the 'Title Tag'.
As tmongy says, it will not affect your PR but it can affect your placement in the SERPs for some keywords.
These keywords will be only those which are related or influenced somehow by your current title, it won't affect the others
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i ever change my title tag and it not effect my pagerank or ranking on google
PR relates to how many backlinks you have from higher-PR level sites. Don't worry if you want to change some of the pages content or title tag, this won't affect your PR at all.
I WOULD worry more about how changing the <title> and such will change your URL rankings for targeted keywords.
If your URL already ranks well then I would be careful how you proceed. I would change them VERY slowly and watch for a few weeks to see how rankings are affected. If negatively then change back. If positively, make another minor change and wait a few weeks to see how rankings are affected.
If your URL ranks poorly then you likely can't make it much worse. Have at it making lots of changes at once if you want.
when i changed my blog's title, my pagerank not decreased