I have been taken antibiotics for chronic prostatitis for two monght, the pain in urethra was eliminated ,but I still have pain in lower back and in the urethra.

Antibiotics can treat prostatitis in a short period. If you still have symtpom after taking antibiotics for long, you should try alternative treatments. 
If you're interested in herbal remedy, I want to recommend Diurtic and Anti-inflammatory Pill. It is a kind of traditional Chinese medicine made from almost 50 kinds of elements and each of them have their unique function in curing genital system and urinary system disesases. Some of them can promote blood circulation and dissolve stasis, some can promote Qi and stop pain, some can kill bacteria, virus, mycoplasma and chlamydia, and some can clear away heat and dampness. They all aiming at eliminating your symptoms to cure it.                                    
Released in 2011-11-10 11:19:03