Phra Pathommachedi or Phra Pathom Chedi is a stupa in Nakhon Pathom, Thailand. The stupa is located in the Wat Phra Pathommachedi Ratcha Wora Maha Wihan, a temple in the town center. Phra Pathommachedi is the tallest stupa in Thailand. The top of its spire reaches 120.45 metres, with the base circumference of 235.50 metres.
The name Phra Pathommachedi means the first holy stupa, given by king Mongkut. Originally the stupa was named Phra Thom Chedi meaning the big stupa in the ancient Khmer language or the royal stupa in Northern Thai language. It is the common belief that this stupa is the oldest and the first stupa in Suvarnabhumi, the ancient name of a civilisation in Southeast Asia. Modern historians believe that the stupa was one of the principal stupas of ancient Nakhon Pathom, the largest settlement in the Dvaravati period in the Nakhon Pathom area together with the nearby Phra Prathon Chedi during the 6th to the 8th centuries.
From the Dvaravati Period there is no historical record regarding the origin of Phra Pathommachedi, but according to Subhadradis Diskul, a prominent Thai historian and archaeologist, Ashoka, an Indian Emperor who ruled almost all of the Indian subcontinent from circa 269 to 232 BCE, sent a prominent Buddhist monks to expand Buddhism in Suvarnabhumi including the area that Nakhon Pathom is in today thus creating many legends about the stupa's construction.
During Khmer sovereignty in the 11th century, the chedi was modified with the addition of a Khmer style prang but following invasion of the city by the Pagan Kingdom, the Phra Pathommachedi was abandoned. During the Sukhothai and Ayutthaya Periods, no attempt was made to restore the chedi as the regional centre of power was in Nakhon Chai Si. It was left to King Mongkut (Rama IV) after his coronation to restore the chedi by building a new chedi in Sri Lankan style covering the old one. In the latest development, in 1907 as crown prince Vajiravudh (later Rama VI), after visiting Phra Pathommachedi several times decided to build Sanam Chandra Palace in Nakhon Pathom. His ashes and that of his family are kept at Phra Pathommachedi.
In 2009 the Department of Fine Arts, Ministry of Culture announced a plan to promote Phra Pathommachedi to be a UNESCO World Heritage Site because of the stupa's long history and its significance in Buddhist expansion in Southeast Asia and on the Dvaravati culture.
The architecture of the Phra Pathommachedi complex consists of a number of eliments: Phra Pathommachedi, Northern Viharn, Eastern Viharn, Southern Viharn, Western Viharn, Ordination Hall and Meditation Hall. Today it is a place of pilgrimage for many Thai people.

Source: Wikipedia