Power flow angles (PFAs) on a langasite (LGS) substrate with Euler angles of (0°, 138.5°, Ψ), Ψ= 25.7 to 27.7° are investigated for surface acoustic wave (SAW) filters using slanted finger interdigital transducer (SFIT) by an electrical and optical methods. In the electrical method, several tilted SFIT SAW filters with different tilt angles for (0°, 138.5°, Ψ) LGS substrates were designed, and the frequency responses of the filters were measured. In the optical method, the PFAs were directly measured by optical probing for a parallel interdigital transducer (IDT) with wide propagation area on the substrate. As a result, a good correlation between electrical and optical measurements of the PFAs is obtained, but the calculated PFAs are slightly different from the measured PFAs. A good frequency response of a tilted 380 MHz SFIT SAW filter with an appropriate tilt angle corresponding to the PFA on the substrate is obtained even though the aperture is small.