Introduction A prolonged mismatch between the demand for respiratory muscle work and the capacity to meet that demand in chronic obstructive pulmonary disease (COPD) can result in symptoms of dyspnea and the development of muscle fatigue. Objectives The purpose of this study was to compare two different respiratory muscle endurance assessment methods of sustainable inspiratory pressure (SIP) and incremental threshold loading (ITL) in COPD. Methods Thirty-four patients diagnosed with COPD were included in the study. Physical and demographic characteristics of the subjects were recorded. Pulmonary function testing (PFT), modified Medical Research Council (MMRC) dyspnea scale, COPD assessment test, inspiratory and expiratory muscle strength (MIP and MEP, respectively) measurement and six-minute walk test (6MWT) were used to assess subjects. Two different respiratory muscle endurance tests (SIP and ITL) were performed within 48 hours apart, and each test was repeated two times on the same day. Results There was no correlation between the SIP and ITL tests (r= 0.069,P= .699). According to the test-retest reliability analysis, both tests were reproducible (ICC = 0.843;P< .001 for SIP, ICC = 0.905;P< .001 for ITL). Finding no differences between repeated tests showed that tests are not affected by learning effect. Conclusion The SIP and ITL tests are used for the same purpose but have different characteristics. Regarding the relationship between the other parameters and tests, the ITL is well tolerated and reflects the differences in respiratory muscle endurance better.