Yawata Maru was the second of three ships of the Nitta Maru-class and was built by Mitsubishi Shipbuilding & Engineering Co. at their Nagasaki shipyard for Nippon Yusen Kaisha (NYK).

Between 25 November and 31 May 1942, Yawata Maru was converted into an escort carrier at Kure Naval Arsenal and she was renamed Un'yō on 31 August.

Un'yō carried enough fuel oil to give her a range of 8,500 nautical miles (15,700 km; 9,800 mi) at a speed of 18 knots (33 km/h; 21 mph).

[12] Un'yō made three voyages from Japan to Truk, Saipan, and Rabaul between July and October 1942, during which she delivered 10 Mitsubishi A6M Zero fighters to the latter on 11 September.

The ship departed Yokosuka on 28 October and picked up the 11th Fighter Regiment at Surabaya, Java, on 2 December and delivered them to Truk on 11 days later.

[13] The carrier continued her transport missions between Japan and Truk for the rest of the year, with a brief refit at the Kure Naval Arsenal from 30 September to 9 October.

To reduce pressure on the interior bulkheads, the ship's captain began steaming in reverse at half speed towards Yokosuka.

While sheltering at Garapan Anchorage, Saipan, to conduct emergency repairs, a follow-up attack by Halibut on 23 January was driven off.

The convoy encountered a bad storm two days later and part of her damaged bow snapped off, causing the forepart of the flight deck to collapse.

To prevent further damage, Un'yō began to steam stern-first and arrived at Yokosuka on the 8th to begin permanent repairs that were completed by 28 June.

Un'yō carried ten Nakajima B5N "Kate" torpedo bombers and six Kawasaki Ki-3 biplane training aircraft of the 91st Squadron on this voyage.

A storm developed during the night and the force of the waves pounding on the carrier's stern collapsed the interior bulkheads despite them having been shored up.

The carrier sank 25 minutes later, 220 nautical miles (410 km; 250 mi) southeast of Hong Kong, at 19°08′N 116°36′E / 19.133°N 116.600°E / 19.133; 116.600, after 761 survivors had been rescued by two of the escorts.