Background: Thoracoscopic surgery is an invasive surgical technique used to diagnose and treat problems in the chest. Since most patients are unfamiliar with the thoracic surgical procedure, the amount of information may be overwhelming, especially if patients receive a new cancer diagnosis or are unaware of the scope or hazards of the anticipated surgery. On the day of operation, it is frequently necessary to reiterate the initial consultation. So, the current study aimed to evaluate the effect of instructional guidelines on patients' knowledge, practice, and stress regarding thoracoscopic surgery. Design: This study was carried out using a quasi-experimental research design. Setting: This study was applied in the thoracic surgery department at Damanhour university hospital. Subjects: It consisted of a purposive sampling technique enrolled to select a sample of fifty patients undergoing thoracoscopic surgery in the previously selected department who agreed to participate in the study and were randomly assigned into two equal groups, with 25 patients in each group (the experimental group who received instructional guidelines and the control group who received routine hospital care only). Tools: Patients' interviewing sheet, patient's knowledge questionnaire, patients' practice assessment sheet, and Cohen's Perceived Stress Scale Short Version (PSS-10). Results: There was a highly statistically significant difference between the study group patients' pretest and posttest in total knowledge and practice after the implementation of instructional guidelines. There was no statistical significance reducing the difference between both groups as regard stress scores. Conclusion: There was an improvement in total knowledge, practice, and reducing stress after instructional guidelines implementation with higher percentages in the study group than in the control group. Recommendations: instructional guidelines should be given for patients to improve their knowledge, practice, and reduce stress regarding thoracoscopic surgery.