The report assessed the outcome of funds contributed by the Manila-based bank's wealthy Western members to the operation of its Asian Development Fund.
The fund provides concessional loans to its poorest member countries at minimum or no interest.
The report said the administration of some USD 5.4 billion worth of concessional loans approved between 2001 and 2004 was "less than efficient" and added that there was a "decline in effectiveness in the period."
The study was released amid reports that major donor nations to the ADB are demanding that the bank overhaul its operations amid growing disillusionment with the bank's slow pace of reform.
Concerns about the ADB have prompted the UK to withdraw a commitment to provide more funds, citing a "lack of significant progress on the reform agenda", the Financial Times newspaper reported on Thursday.
The in-house report said sometimes conflicting demands by donors as well as recipient governments meant less of the money went to projects meant to advance the United Nations' Millennium Development Goals of reducing global poverty, "such as health and agriculture, and to microfinance."
It said, "efficiency has been improving" since 2005, and the adjustments made should ensure some seven billion dollars of soft loans allotted for 2005-2008 are "likely sustainable in terms of effective poverty reduction."
The report urged the bank to "avoid goal congestion in operations and in ADB as a whole" because supervision and resources were spread too thin.