* Achieves net return of 9.7 pct in 2017
* 18.8 percent return from private equity investments
* 18.2 percent return from infrastructure (Adds comment on strategy, details on asset classes)
TORONTO, April 3 (Reuters) - Ontario Teachers' Pension Plan, Canada's third-biggest public pension plan, more than doubled its 2017 net return from investments compared with the year before, boosted by strong returns in private equity and infrastructure.
The fund, which administers pensions for 323,000 working and retired teachers in Canada's most populous province, reported a net return from its investments of 9.7 percent in 2017, compared with 4.2 percent in 2016, when unfavorable currency movements had an impact.
Ontario Teachers', one of the world's biggest investors in infrastructure and real estate, has benefited from a strategy of buying undervalued assets whose performance can be improved.
"This year we lowered our exposure to passive investments and moved further into active strategies where we can add value by leveraging our experience as well as our capital," Chief Executive Ron Mock said in a statement.
Returns from private equity investments totaled 18.8 percent during the year while infrastructure investments achieved an 18.2 percent return.
Strong performances from those asset classes helped the fund increase the value of its net assets to C$189.5 billion ($148 billion) at the end of 2017, compared with C$175.6 billion 12 months earlier.
The fund also said Chief Investment Officer Bjarne Graven Larsen, who joined Ontario Teachers' in January 2016 and has steered its investment strategy since then, had resigned and planned to return to Denmark with his family.
Mock, who will become interim CIO while the fund searches for his replacement, said he did not expect any significant change to the fund's strategy following Larsen's departure.
"We've gone through a transition where the portfolio has been modified and adjusted to a more risk-balanced approach. That's been in flight under Bjarne for the last couple of years," he told reporters. "As we roll forward I'm not expecting any significant change."
($1 = 1.2795 Canadian dollars) (Reporting by Matt Scuffham Editing by Susan Thomas)