This study shows that exposing mushroom powder suspended in ethanol to ultraviolet light effectively converts ergosterol (a natural compound in mushrooms) into vitamin D2, which is essential for bone health and immunity. The optimal treatment involves UV-C light exposure for 120 minutes at a specific distance, producing about 15 times more vitamin D2 than direct irradiation of dry mushroom powder. After removing the ethanol, these treated mushrooms could serve as a practical natural source of vitamin D for people with deficiency.