Objectives: This study was aimed to compare the retention among 3 types of orthodontic aligners according to their thickness and according to the presence of beveled attachments. Materials and methods: An impression was taken for the upper arch of a patient, poured with die stone and scanned using CS 3600 intraoral scanner. The treatment plan was done using blue sky plan software and 6 models with attachments and 6 without attachments were exported. The first two models of each group were printed using Micro play 3D printer, and 3 thermoplastic materials were used in this study. Total 10 samples for each material were thermoformed over each of the 4 printed casts to make aligners. Retention tests were done on the 2 original models; one with the attachment and another without the attachment using the tensile test in the universal testing machine to measure the maximum forces required to remove the aligner from the model. Results: Leone 0.8 mm was higher than clear aligner (0.5 mm) in the retention tests of all the groups. Leone (0.8 mm) was higher than Duran (1 mm) in 2 groups (aligner number 1 groups). The presence of attachments led to an increase in the retention means of all the 3 brands when compared to the non-attachments group. Conclusion: Clear aligner (0.5 mm) is the least retentive aligner. Leone (0.8 mm) has the highest retention value in both attachments and non-attachments groups regarding aligner number 1. Adding beveled attachments lead to the significant increase in the retention of all aligners regarding the aligner number 1.