The World Bank has announced a pilot program in which it will enter into joint loan agreements with commercial banks for Third World development projects.

These new "co-financing" deals will allow the bank to invest up to $500 million in the next two years in direct participation in loans with commercial banks. Until now, co-financing with commercial banks has been limited to separate loan agreements with the borrower--one by the commercial bank or consortium on market terms, the other by the World Bank.

Bank Vice President Ernest Stern said last week the bank hopes its direct participation--generally limited to 20 percent of the commercial transaction--will help increase the flow of private capital to the Third World. In addition to the direct participations, the bank will continue to make direct loans of its own in co-financed projects. tern said that the pilot phase will cover 15 to 20 lending operations, and with the 20 percent ratio, the bank's $500 million could result in total co-financing of about $2.5 billion.

In announcing three basic separate "instruments" or options for the new co-financing operation, the bank's board of executive directors said the funds "will be used to finance high quality investment projects already deemed suitable for World Bank lending in those countries which the bank judges to be credit-worthy."

The three options, each designed to achieve "significantly longer maturities" than available in the commercial market, require that:

* World Bank participation would come in the final years of the loan. Thus, the commercial banks would be paid-off first. As the loan is amortized, the World Bank would make its share available for sale to the private banks.

* Instead of direct participation in the later years, the World Bank would take a contingent responsibility in an arrangement in which the annual debt service charged the borrower is fixed, even though the rate is variable. Thus, if interest rates rise, the World Bank agrees to finance the balance due at the end of the initial term.

* Instead of direct funding, under this option the World Bank guarantees repayment of the final years of a loan.

Some World Bank member countries that would be eligible for co-financing have expressed concern that the monies thus made available to them would be a substitute for the bank's ordinary loans. But Bank President A. W. Clausen said in a recent interview that "co-financing should be additive, and not a substitute."

From the standpoint of the commercial banks, co-financing provides them with the security "umbrella" of association with the World Bank, encouraging them to make loans they might otherwise consider risky.

The co-financing approach has been used more widely by the bank in recent years, with governments as well as banks acting as co-lenders. The total volume co-financed rose from $496 million for 37 projects in fiscal 1973 to $7.4 billion for 103 projects in fiscal 1982. In the same span, total project costs (including money put up by the borrowers) increased from $2.8 billion to $23.3 billion.