In Chiang Mai you can watch muay thai fights at Chiang Mai Boxing Stadium or Tha Phae Boxing Stadium. They aren't as large as Bangkok's Lumpinee or Rajadamnern, but the bouts are still spectacular.
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AddressTha Phae Boxing Stadium, 1 Mun Mueang Rd Old City Phra Sing, Mueang Chiang Mai District, Chiang Mai 50100, Thailand