油滴实验（Oil-drop experiment），是罗伯特·密立根与哈维·福莱柴尔（Harvey Fletcher）在1909年所进行的一项物理学实验．罗伯特·密立根因而获得1923年的诺贝尔物理学奖。

此实验的目的是要测量单一电子的电荷。方法主要是平衡重力与电力，使油滴悬浮于两片金属电极之间。并根据已知的电场强度，计算出整颗油滴的总电荷量。重复对许多油滴进行实验之后，密立根发现所有油滴的总电荷值皆为同一数字的倍数，因此认定此数值为单一电子的电荷e：

到2006年为止，已知基本电荷值为1.60217653(14) x 10库仑。密立根在诺贝尔奖颁奖典礼上，表示他的计算值为4.774(5) x 10静库仑（等于1.5924(17) x 10库仑）。现在已知的数值与密立根的结果差异小于百分之一，但仍比密立根测量结果的标准误（standard error）大了5倍，因此具有统计学上的显著差异。

The oil drop experiment was performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the elementary electric charge (the charge of the electron).

The experiment entailed observing tiny charged droplets of oil between two horizontal metal electrodes. First, with zero applied electric field, the terminal velocity of a droplet was measured. At terminal velocity, the drag force equals the gravitational force, and these depend on the radius in different ways, so that the radius of the droplet, and therefore the mass and gravitational force, could be determined (using the known density of the oil). Then an adjustable voltage was applied between the plates to induce an electric field, and the voltage was adjusted until the drops were suspended in mechanical equilibrium, indicating that the electrical force and the gravitational force were balanced. Now using the known electric field, Millikan and Fletcher could determine the charge on the oil droplet. By repeating the experiment for many droplets, they confirmed that the charges were all small integer multiples of a certain base value, which was found to be 6981159240000000000♠1.5924(17)×10 C, less than a 1% difference from the currently accepted value of 6981160217648699999♠1.602176487(40)×10 C. They proposed that this was the (negative of the) charge of a single electron.