VANCOUVER, British Columbia (AP) _ Canadian Greenpeace members say they will try out a giant driftnet held aloft by balloons to determine whether it can be used to stop free-flight tests of the American cruise missile.

The protesters plan to be in northern Alberta on Tuesday, stationed along the 11-mile-wide flight path to be followed by the missile, which will be mounted on a B-52 bomber on a run that will take it over the northeastern corner of British Columbia.

Greenpeace spokesman Jim Bohlen said at a news conference Monday that the driftnet, which measures 100 feet by 25 feet, will be flown over Highway 63 in northern Alberta and will be returned to the ground before the B-52 arrives.

He said the protesters plan to use sighting devices to fix the elevation and flight path of the missile.

Bohlen said Greenpeace then plans to invite other peace groups to erect nets of their own when free-flight cruise tests are scheduled for later this winter. He said scientists and military sources say a driftnet could be an effective block to a cruise missile because, if contact was made with the net, fragments would probably enter the engine and cause a malfunction.

Bohlen said he hopes the presence of the nets would force the authorities to cancel the tests or force the missile to ''avoid the net which would interfere with the purpose of the tests.''

Bohlen said chances of getting the nets in the cruise flight path are ''pretty good.''

He said Greenpeace doesn't want to harm anybody but the environmental group is willing to take that risk.

The B-52, actually carrying a cluster of four of the cigar-shaped missiles, not equipped with nuclear warheads, will take off from an air base in Grand Forks, N.D., and enter the test corridor between 9:30 a.m. and noon EST. It will return to Grand Forks after the 41/2 -hour test without landing in Canada.