I have 3rd degree acne and I am taking isotretonoin tablets 20 mg for past 1 month but no effect? Can I continue or any other better treatment?

Accutane. Your dose is way to low. The goal is to prevent recurrance of acne. At this dose there is a good chance that it will not work and an even better chance that the acne will return when you stop the medication.
Not working. One month is not sufficient to give this medication a good chance. Most patients have less new lesions and some healing in the first month, almost no new lesions and more healing, and by the third to fourth month very few new lesions, and significant healing of the older acne lesson. You may need to retry some other combinations with your dermatologist if this does not happen.