Generally no if the policy is an individual policy where premiums are not deducted from income for income tax purposes. The benefit is received in this condition as a non-taxable benefit paid to you by the insurance company.
If the plan is part of a group plan or the premiums have been deducted under special circumstances, then the benefit would be taxable.
Generally, critical illness insurance is not a taxable benefit. The money that is used to purchase critical illness insurance has already been taxed, since critical illness insurance is not tax exempt as a medical expense. Thus, the benefits paid if the critical illness policy pays out, are not taxable.
Critical Illness Insurance premiums are usually paid with after tax earnings, at least if the policy is owned by an individual. When insurance is bought with after tax money, the benefit received from that policy is usually tax free. Therefore, collection of Critical Insurance benefits is not generally a taxable event.
If the plan is part of a group plan or the premiums have been deducted under special circumstances, then the benefit would be taxable.