Hard particles under the skin can be caused by acne, folliculitis, and fat grains. 1. Acne: The exuberant secretion of androgen in the body causes an increase in the oily secretion of the hair follicles, which may cause the secretion under the skin of the hair follicles to be discharged poorly, forming localized acne and leading to hard particles under the skin. 2. Folliculitis: secondary bacterial or fungal infection of the hair follicles, resulting in congestion and swelling of the hair follicles, which may also lead to follicles with hard particles under the skin. 3. Fat grain: embodied fat metabolism is abnormal, may also be manifested as subcutaneous fat, particles caused by more or less smooth and soft, good mobility granular material. It can also cause hard particles under the skin. There are hard particles under the skin, there may be other reasons, it is recommended to go to the hospital in a timely manner to check and clarify the cause of the disease on the treatment prescribed by the doctor.