Background: Stereotactic Radiotherapy utilizes well-collimated and highly confined beams to treat the small targeted lesion demanding high levels of precision and accuracy in QA procedures. In this study, the RUBY modular QA phantom was evaluated for isocenter verification and pretreatment dose verification of SRS/SRT treatments. Material and Method: RUBY modular QA phantom was used for daily QA checks of geometric accuracy of radiation isocenter carried out with linac QA insert. Data was acquired for 100 days with Ruby phantom for TrueBeam linac. Isocenter verification was also done by IsoCal phantom and results were compared with Ruby phantom results. Patient-specific QA was performed for 25 SRS/SRT treatment plans using a patient QA insert with a pinpoint 3D ionization chamber as a detector. Results: The maximum diameter of the isocenter sphere is 1.04±0.12mm, 0.99±0.13mm, and 1.07±0.22mm for the gantry, collimator, and couch. The average isocenter offset is 0.45±0.18mm, 0.40±0.2mm, and 0.23±0.2mm for the gantry, collimator, and couch measured. The average isocenter deviation measured with the IsoCal is 0.35±0.02mm, in close agreement with the value 0.41±0.11mm measured with Ruby phantom. The point dose values accessed by the RUBY phantom agree to the expected dose within the acceptance limit of 3%. Conclusion: Ruby phantom is consistent, rapid, and easy to implement for daily QA checks and its results proved to agree with IsoCal phantom. RUBY phantom is also recommended for pretreatment QA SRS/SRT, using patient QA insert. The modular construction of the RUBY allows using of a single phantom to perform different QA processes for a more synchronized and harmonic QA workflow using a variety of task-specific inserts.