Castillo (5-1) racked up five-straight wins to open his career but was defeated by Donald Cerrone via first-round submission in his WEC debut at WEC 34 in June.
Ratcliff (6-1) followed up four-straight victories outside of the octagon with back-to-back victories in the WEC over Johnny Sampaio and Alex Karalexis. He was defeated by Marcus Hicks via first-round submission in his most-recent bout at WEC 33 in March.
The bout will appear on the unaired preliminary card.
WEC 36 takes place on September 10th at the Seminole Hard Rock Hotel and Casion in Hollywood, Florida and features WEC featherweight champion Urijah Faber putting his title on the line in the main event against Mike Brown.