But that is unlikely to continue, Mr Tangen told Reuters.
"Now we have record low interest rates and record-high stock markets. With record-low interest rates, they are unlikely to go down further," Tangen said, adding that the relationship between risk and return was now different, for both bonds and stocks.
"So we are extremely unlikely to have any kind of repeat of the last 25 years," he added.
When it first became a sovereign wealth fund it only invested in government bonds, with corporate bonds, stocks and real estate added later. Last year, the fund was allowed to take direct stakes in renewable projects.