What is the location of prostate gland in human male reproductive system?

Below seminal vesicle.

Prostate gland is a large chestnut shaped gland which is located below seminal vesicle.

The primary function of the prostate is to contribute prostatic fluid to semen. 

During ejaculation, the prostate contracts and squirts prostatic fluid into the urethra. 

The prostate contains an enzyme called 5-alpha reductase, which converts testosterone into a biologically active form called dihydrotestosterone (DHT).