J R R Tolkien had the post of Professor of English at Oxford for many years. Can anyone tell me what was his PhD thesis entitled?

This webpage at the Tolkien Society states he did not write a PhD thesis.

http://www.tolkiensociety.org/faq01.html#awards

On the academic front, Tolkien never "took a Ph.D." was we now sometimes say - he was too busy working professionally on the kind of stuff people normally do Ph.Ds on - but he was awarded a Doctorate of Letters (D. Litt.) and Philosophy by the University of Liege in Belgium in 1954 and similarly a D. Litt by the University of Dublin in Ireland that same year. In both cases this was for his contribution to his field of philology and medieval literature in general, and his services to the universities in particular as a contributing examiner and researcher.

In any case, the degree of Doctor of Philosophy was very new in Britain at the time, first becoming available in 1917. Although it soon proved popular in the natural sciences, the same was not the case in the humanities. (Only about half the people who taught me at university had a doctorate).

https://en.wikipedia.org/wiki/Doctor_of_Philosophy