Random error is also called the accidental error, which is the error with absolute value and symbol changing in unpredictable ways occurring during measuring the same value for many times under the same conditions. Random error mainly is caused by the comprehensive influences to the measured value due to independent random factors (for example, micro variation of electromagnetic field and air disturbance and the earth micro-quake tremors, etc.) during process of measurement. The size and direction of random error in single measurement are unpredictable, so it cannot be modified and cannot be eliminated with experimental method. But the random error is subordinated to the statistical regularity in the totality of many times of measurement, so its effects may be estimated by means of probability theory and mathematical statistics method.