碳氮氧循环（CNO cycle），有时也称为贝斯-魏茨泽克-循环（Bethe-Weizsäcker-cycle），是恒星将氢转换成氦的两种过程之一，另一种过程是质子-质子链反应。

在质量像太阳或更小些的恒星中，质子-质子链反应是产生能量的主要过程，太阳只有1.7%的氦核是经由碳氮氧循环的过程产生的，但是理论上的模型显示更重的恒星是以碳氮氧循环为产生能量的主要来源。碳氮氧循环的过程是由卡尔·冯·魏茨泽克和汉斯·贝特 在1938年和1939年各别独立提出的。

碳氮氧循环的主要反应如下：

这个循环的净效应是4个质子成为一个α粒子、2个正电子（和电子湮灭，以γ射线的形式释放出能量）和2个携带着部分能量逃逸出恒星的微中子。碳、氮、和氧核在循环中担任催化剂并且再生。

The CNO cycle (for carbon–nitrogen–oxygen) is one of the two (known) sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction. Unlike the latter, the CNO cycle is a catalytic cycle. Theoretical models suggest that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. The proton–proton chain is more important in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions starts at temperatures around 7006400000000000000♠4×10 K (4 megakelvins), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts at approximately 7007150000000000000♠15×10 K, but its energy output rises much more rapidly with increasing temperatures. At approximately 7007170000000000000♠17×10 K, the CNO cycle starts becoming the dominant source of energy. The Sun has a core temperature of around 7007157000000000000♠15.7×10 K, and only 7000170000000000000♠1.7% of 4He nuclei produced in the Sun are born in the CNO cycle. The CNO-I process was independently proposed by Carl von Weizsäcker and Hans Bethe in 1938 and 1939, respectively.