I searched too much but can't find any salution over the internet.
label=QLabel("hello",self)
label.setStyleSheet("QLabel{font-size:50px;font-family:'Orbitron'}")

OR

from PyQt5.QtGui import QFontDatabase label=QLabel("hello",self)
QFontDatabase.addApplicationFont('file_name.otf or .ttf')
label.setStyleSheet("QLabel{font-size:50px;font-family:'Orbitron'}")
How to import font family in pyqt5?