Belarus:
The executive power in Belarus is extremely centralized, even authoritarian. The president is dominating over the political system and is the only one that is really running the country.
The position of the President, who is Alexander Lukashenko, consists of a wide range of executive powers. He serves as both the head of state and the head of the government.
The president appoints the main governmental officials, including the prime minister, who is primarily not holding any power and is being put in the position just in case something will happen to the Lukashenko himself. The president can directly influence the legislative process, that is another power he has.
An order issued by the president carries the force of law without any legislation.
The president has control over the judiciary. The judiciary is highly dependent on the president. The bureaucracy in Belarus operates under the authority of the president. All of the bureaucratic positions are typically filled by individuals loyal to the president, making the authoritarian nature of the government even stronger.

Ukraine:
At the same time, Ukraine has a semi-presidential system where executive power is more divided, at the same time the president remains an important figure in Ukrainian politics. The country has been thru several political reforms in order to achieve decentralization of power, especially during the 2014 Euromaidan Revolution.
The President of Ukraine is the head of state and the head of government. The president is directly elected by the people for a five-year term.
The president has a wide range of powers including appointing the prime minister, the cabinet, and key decision maker on military and foreign policy decisions.
The president can veto legislation, even though this can be overridden by the Verkhovna Rada.
The prime minister has a more substantial role in the daily practice of government, especially in domestic policy and administration, than in Belarus.
Prime Minister is appointed by the president but has to be confirmed by Verkhovna Rada. The head of government and the prime minister are responsible for coordinating activities of the government and the supervision of public administration.