Wat Phra Chao Phya-thai, populary known as Wat Yai Chai Mongkol, is situated to the southeast of the city. The large chedi there can be seen from a great distance. This monastery was built by King U-Thong which exhumed the bodies of Chao Kaeo and Chao Thai who died of cholera, and cremated in 1357 A.D. The monastery was dedicated to the monks who had gone to study practical Buddhism in Ceylon. As the group gained the popularity, the King then appointed the head of the sect to become Somdet Phra Wannarat or the right supreme Patriarch. The temple was finally rebuilt in memorial of King Naresuan the Great for his victory over the Burmese.
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Considering its architectural design, experts believe the stupa was built even before Ayutthaya was founded. King Naresuan the Great had it restored and built some additions to commemorate his great victory in battle over the Burmese. He named it "Chai Mongkol" or "Auspicious Victory".