Two-dimensional (2D) materials play key role in designing and fabricating diminutive optoelectronic devices with high efficiency. In this paper, we report the results of a comprehensive first-principles study on the structural and electronic properties of the pristine and hydroxyl group OH-functionalized (OH-AlN-OH) AlN monolayer. GGA-PBE and hybrid HSE06 functionals are employed to describe the exchange-correlation potential. According to our calculations, the pristine AlN monolayer has a wide indirect band gap of 2.954(4.000) eV determined by PBE(HSE06) level of theory. Indirect-direct gap transition is obtained through the chemical functionalization and the band gap reduces to 0.775(2.125) eV. Results shows that the OH-AlN-OH monolayer is more suitable for optoelectronic applications. Finally, the strain is proven to be efficient factor to tune the electronic properties of the studied monolayers.