  Theta vacuum effects on the QCD phase structure in the \mu-T plane are
studied by using the Polyakov-loop extended Nambu-Jona-Lasinio model and its
extension, where \mu is the quark chemical potential and T is temperature,
respectively. As the parameter \theta of the theta vacuum increases, the chiral
transition becomes stronger. For large \theta, it eventually becomes first
order even at zero \mu.
