Stress generated by thermal annealing of thick parylene-n (PA-n) films (4–7 μm) has been studied. We examined the behavior of stress for as-deposited films and rapidly thermally annealed films. The annealing was done in vacuum and the stress measured after the anneal. In another experiment the stress was also studied in-situ with thermal annealing. The as-deposited films are under compressive stress of about 18 MPa. When the backside film was removed by plasma etching, the stress changed from compressive to tensile. As the films are annealed, the stress decreases and then becomes tensile, peaking at about 60 MPa. The film crystallinity and stress are correlated for PA-n films. It was observed that stress is reduced during the crystal phase transformations at around 220°C and 270°C. This property can be used to relieve stress in multilevel structures. It was also found that slower heating and cooling led to lowered stress. The stress has been compared to reported values from other sources. Stress in PA-c (chlorinated) and co-polymerized PA-n and -e (esterfied) films has also been reported.