My dad (50) is taking clonazepam 0.5 mg for anxiety for 20 years..now it's not well effective..please prescribe any other medicine for him?

Agree with you.: He may need to increase or consider adding or switching to another med.

Suggestions: Will need to talk to doctor where you are for prescription, but glad to make suggestions. Clonazepam is a benzodiazepine which leads to tolerance meaning dose needs to be increased for ongoing effect. 0.5 mg is a very low dose, but increasing dose is not the answer. Many alternatives: buspirone, paroxetine, citalopram, venlafaxine, duoxetine, just to name a few.