To study the pulse-broadening mechanism in an injection mode-locked TEA CO2 laser oscillator, a train of Gaussian pulses with variable widths were injected in the slave-oscillator. Theoretical analyses and experimental data are presented of the AM mode-locked master-oscillator with an additional c.w. low-pressure gain cell in the cavity and of the broadening mechanism on the injected pulse in the slave-oscillator. Injection mode-locking is now a commonly used method for generating high-energy nanosecond pulses. Due to the absence of inserted modulators, which have in general relatively low damage thresholds and small apertures, this technique allows the generation of high-power short pulses.