SUN is the smallest unit of TRX, TRON's native token: one millionth of a TRX, or 0.000001. Every balance and every fee on TRON is stored and computed as a whole number of SUN.
The short answer
One TRX equals exactly 1,000,000 SUN (10⁶). Six decimals is unusually coarse by crypto standards (Ethereum uses eighteen, Solana nine), but it is plenty for TRON's fee scales, and it happens to match the six decimals used by USDT, the token that dominates the network's traffic.
As with every chain's base unit, the point is integer arithmetic: TRON nodes account in whole SUN so that consensus never depends on floating-point rounding. The decimal display is applied at the edge, by wallets and explorers.
Named after Justin Sun
The unit is named after Justin Sun, TRON's founder. Where Ethereum and Solana name their denominations after cryptography pioneers (Wei Dai, Hal Finney, Leslie Lamport), TRON's founder put his own name at the bottom of the unit ladder. Make of the tradition what you will; the arithmetic works the same.
One disambiguation matters in practice: there is also a SUN token, the governance token of the sun.io DeFi platform on TRON. The unit and the token share a name and a blockchain but have nothing else in common. If something is priced "in SUN" in fee documentation, it means the TRX subunit, not the token.
Where SUN shows up
SUN is the pricing unit for TRON's two fee resources. Energy, consumed by smart-contract execution, is currently priced at 420 SUN per unit, and bandwidth, consumed by transaction bytes, at 1,000 SUN per byte. Both constants are set by committee vote and have changed before, which is why serious fee math always keeps the SUN price as a variable. Energy vs bandwidth explains the whole model.
A worked example: a standard USDT transfer to an address that already holds USDT consumes about 31,895 energy and 345 bandwidth. In SUN that is 31,895 × 420 + 345 × 1,000 = 13,740,900 SUN ≈ 13.7 TRX. Numbers like that are why the TRON fee calculator works internally in SUN and only converts to TRX and USD at the end.
Why six decimals is enough
A base unit only needs to be small enough that the minimum meaningful price is still a whole number. TRON's cheapest metered resource costs hundreds of SUN, so one SUN of resolution leaves two orders of magnitude of headroom. Contrast Ethereum, where gas prices needed to express billionths of an ETH, hence 10¹⁸ wei. Different fee markets, different atom sizes, same integer-only principle underneath.