In this paper, a micromachined silicon straight tube is tested as a fluid density sensor. In comparison with other density measurement techniques, the use of micromachined tubes require small sample volumes and allows continuous monitoring of the fluid density in microfluidic systems. Different vibration modes of the sensor were detected and calibrated using a laser Doppler vibrometer (LDV). Linearity, simplicity, the straightforward fabrication and evaluation, the low flow restriction and reduced risk of trapping gas in the sensor due the absence of corners are the design's main advantages. The ability of the sensor to measure density of multiphase fluids and provide accurate results independent of other fluid parameters, allows it to be used in varying fields such as the biomedical, pharmaceutical and petrochemical industries.