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Sri Lanka is the oldest continually Buddhist country, Theravada Buddhism being the major religion in the island since its official introduction in the 2nd century BC by Venerable Mahinda, the son of the Emperor Ashoka of India during the reign of King Devanampiya- Tissa. Later, the nun Sanghamitta, the daughter of Asoka, was said to have brought the southern branch of the original Bodhi tree, where it was planted at Anuradhapura. From that day up to the present, the Buddhists in Sri Lanka have paid and are paying the utmost reverence to this branch of the Bodhi Tree under the shade of which the Master achieved Enlightenment.
It was in Sri Lanka, in the 1st century AD during the reign of King Vatta Gamini that the Buddhist monks assembled in Aloka-Vihara and wrote down the Tripitaka, the three basket of the Teachings, known as the Pali scriptures for the first time. It was Sri Lankan nuns who introduced the Sangha of nuns into China in 433AD.
According to the Sri Lankan chronicles, the Mahavamsa, one of Ashoka’s sons, the monk Mahinda, supervised construction of monastic buildings near Anuradhapura. Simultaneously, he sent to India for relics. These, say the histories, included the Buddha’s alms bowl and his right collarbone. Later a hair relic, and in the 4th century AD, the Buddha’s tooth would be taken to Sri Lanka. The tooth is still preserved in Kandy where daily rituals venerate the Buddha’s tooth relic in Temple of the Tooth Relic, Kandy 16th Century.
To house the relics, stupas were built. Standing at 300 feet, Ruwanweliseya, or the “Great Stupa” is regarded as one of the most important stupas at Anuradhapura in north-central Sri Lanka: Much restored, the great dome, circled with old columns, is still to be seen in Anuradhapura.. During major festivals it is crowded with hundreds of thousands of devotees in family groups, who picnic happily among the ruins and offer puja at the Bodhi tree. There are other important monuments nearby at Mihintale, the site of Mahinda’s first sermon to King Devanampiya-Tissa. The ruins of the later capital at Polonnaruwa (9th century AD onwards), showing Hindu and Mahayana cultic influence, are yet more elaborate.
source - http://www.buddhanet.net/