An optical true time delay line (OTTDL) is a fundamental building block for signal processing applications in microwave photonics and optical communications. Here, we experimentally demonstrate an index-variable OTTDL based on an array of 40 subwavelength grating (SWG) waveguides in silicon-on-insulator. Each SWG waveguide in the array is 34 mm long and arranged in a serpentine manner; the average incremental delay between waveguides is about 4.7 ps, and the total delay between the first and last waveguides is approximately 181.9 ps. The waveguide array occupies a chip area of ∼6.5mm×8.7mm=56.55mm2. The proposed OTTDLs bring potential advantages in terms of compactness as well as operation versatility to a variety of microwave signal processing applications.