He was interested particularly in the Hellenistic period, the origins of Christianity, Roman history and ancient historiography.
In 1930, Kovalev published an article on the formation of ancient Macedonian state under Philip and Alexander the Great.
In 1936, Kovalev published the first comprehensive Soviet textbook on ancient history for high school.
There he authored two chapters, "The rise of Macedonia and the conquest of Asia" and "The philosophy and art of Greece during its heyday".
In the foreword for the Russian translation Kovalev called Robertson's recognition of Jesus' historicity "a serious flaw" and argued to the contrary.