The authors present an empirical study of profile evolution of lithographically defined photoresist (PR) patterns during thermal reflow and apply the findings to diamond microlens fabrication. During PR reflow, a bulge forms at the edge of the PR pattern and propagates inwards as the temperature and PR thickness are increased. An empirical relationship for this propagation is derived. Furthermore, it was found that at a certain reflow temperature and a limited pattern size, there is a minimum initial thickness of the PR pattern for forming spherical lens profiles. Based on these findings, diamondmicrolenses with a diameter of 400 μm and a previously unachieved radius of curvature of over 13 mm were fabricated. This is underpinned by forming PR microlens patterns with a large radius of curvature and transferring the PR patterns through low-selectivity Ar/Cl2inductively coupled plasmaetching.