adverbial N1 rare writtenliterary
思うに — in my opinion
思うに ・ おもうに
A full written guide for 思うに is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See 思うに in real sentences
Jengo shows 思うに the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Advanced Japanese Grammar. Editorial confidence: medium.