It happened about noon in the area of the 19000 block of Highway 530 between Whitehorse Trail and Ramstad Road, officials said.
The woman was flown via medical helicopter to Harborview Medical Center in Seattle, Oso Fire Chief Willy Harper said. She appeared to be alert and conscious at the time.
Arlington Fire Department medics and Snohomish County sheriff's deputies also were called to the scene.
The tractor was being driven by a 51-year-old Arlington-area man, sheriff's spokeswoman Shari Ireton said.
Deputies believe the incident was an accident, she said. There was no evidence of a crime.
View Tractor accident, 4-30-13 in a larger map
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