ETF fund is a transactional open-end index fund, which is also commonly called ExchangeTradedFunds (ETF). It is an open-end fund that is traded on the exchange and has variable fund share. Transactional open-end index fund is a special type of open-end fund, which combines the operating characteristics of closed-end fund and open-end fund. Investors can purchase or redeem fund shares from fund management companies, and at the same time, they can buy and sell ETF shares in the secondary market at the market price like closed-end funds. However, the purchase and redemption must exchange a basket of shares for fund shares or a basket of shares for fund shares. Because of the existence of securities market trading and subscription and redemption mechanism at the same time, investors can carry out arbitrage trading when there is a price difference between ETF market price and fund unit net value. The existence of arbitrage mechanism makes ETF avoid the common discount problem of closed-end funds.