The Swedish energy firm said it is looking for a replacement to take over at the helm following Loseth's departure.
Loseth said: "I have informed the board that I do not wish to extend my contract after 2015. This is for personal reasons. I have notified the Board early in order to provide clarity and to give them sufficient time for recruitment."
Having previously been senior executive vice president with the company, Loseth became CEO in 2010 on a five-year contract.
Vattenfall also announced its results for the first nine months of the year, showing the company swinging back into the black.
Investment in its wind business doubled in the third quarter to SEK 1.1 billion (EUR 126 million), up from SEK 549 million a year before.
Net profit for the group came in at SEK 1.5 billion, compared with a loss of SEK 4 billion previously, on the back of a 10% jump in sales to SEK 37 billion.
This article has been edited to remove - "Wind generation over the nine-month period amounted to 2.5TWh, unchanged from the same period in 2012," after Vattenfall revealed that it's figures are wrong.