Defence Secretary Michael Fallon said the figure was "highly approximate", partly because there were no UK troops on the ground to confirm the impact of the Iraq campaign.
"The estimated number of ISIL fighters killed as a result of UK strikes from September 2014 to 31 August 2015 is around 330. This figure is highly approximate, not least given the absence of UK ground troops in a position to observe the effects of strike activity," Fallon said in a written answer to the UK Parliament. ISIS also goes by another acronym, ISIL.
In September 2014, British MPs had approved British participation in air strikes against ISIS targets in Iraq.
Green Party MP Caroline Lucas had specifically asked Fallon what estimate the US-led global coalition to counter ISIS -- made up of more than 60 countries -- has made of civilian casualties arising from its activities.
However, he was able to give an assessment on the impact of UK action only between September 2014 and August 31 this year.
In August, he had informed the House of Commons that two British ISIS terrorists -- Reyaad Khan and Ruhul Amin -- were killed by an RAF drone strike in Raqqa, Syria.
Last month, Britain announced it was extending its air strikes in Iraq by a year to March 2017.
Newly-elected Labour leader Jeremy Corbyn said British drone attacks were "legally questionable" and called for a "rapid political development" as a way to end the violence in Syria.