In this paper, a new and simple silicon straight tube is tested as a fluid density sensor. The tube structure has a hexagonal cross section. The fabrication process consists of anisotropic silicon etching and silicon fusion bonding. A tube structure with a length of 2.65 cm was tested. The sample volume is 9.3 μL. The first three modes of vibrations were investigated with a laser Doppler vibrometer for air and five liquid mixtures. The fluid density sensitivity of each mode was measured and the average was −256 ± 6 ppm (kg m−3)−1 around the density of water. The density of an unknown fluid can be continuously monitored using this sensor by measuring the resonance frequency of one of the vibration modes and extracting the density from the calibration curves.