Does my Internet Service Provider know if am browsing Porn websites, if i use Tor? because porn is illegal in my country.
No, the ISP does not know what sites you visit using the Tor Browser Bundle.
Have a look at https://www.eff.org/pages/tor-and-https for an interactive infographic of who can see what, and toggle the infographic's "Tor" button there to see the differences.
As long as you don't have any DNS leaks, which should be okay if you're using the recommended settings from the tor browser pack (though can happen in flash and other porn-friendly technologies) you should be okay.
Caution, you may be at risk.
Using Tor your ISP can not monitor the sites you visit but you can be sure that your ISP will Know you are using Tor.
In most countries where pornography is banned, the use of anonymization tools like Tor is also prohibited. It is very possible that the laws of your country punish more severely the use of TOR that viewing pornography.
If you only use Tor for browsing porn websites you should check carefully the laws in your country and assess the use of a VPN instead of Tor ( or both).