Recently viscoelastic measurement has received considerable attention as a complementary method to estimate the crystallization dynamics under shear. However, because of the complex relationship between the crystallization fraction and dynamic mechanical results, it has been difficult to obtain accurate crystallization fraction under shear. To conduct the direct determination of crystallization fraction under shear, the shear flow thermal rheometer (SFTR) having differential thermal analysis (DTA) has been successfully developed using a rotational shear rheometer. Two thermocouples to measure the reference and the sample temperatures were installed at each optimal position as DTA equipment. Two sheets of polyetheretherketone (PEEK) were put on two aluminum plates to prevent heat transfer between the sample and the aluminum plates for accurate measurement of DTA. The influences of using PEEK, instead of an aluminum plate and the thermocouple on rheology results were confirmed to be negligible. In the SFTR, shear strain was given from the lower plate. Both stress and differential temperature responses were simultaneously detected by the upper plate and thermocouples, respectively. The advantage of the SFTR over conventional rheometers was that the crystallinity fraction was able to be directly determined from DTA. In non-isothermal and isothermal preliminary experiments of iso-polypropylene, it was demonstrated that the SFTR provided two kinds of data by the simultaneous measurement of DTA and viscosity. The crystallinity fraction under shear was directly estimated from DTA in the SFTR.