The neodymium YAG laser was used to bleeding experimental house rabbit gastric ulcer in two separate studies: (1) Extensive investigation by using the l. 06 um YAG laser wave lenth, 2 mm in spot size, 30-90 watt power and 0. 5-l. 5 sec pulse duration were performed to determine opitimal YAG laser photocoagulation parameters to maximize hemostatic efficacy while minimizing associated tissue injury. 11 out of 21 combination of power, pulse duration and spot size, with coaxial CO2 were found to be useful. (2) 11 of these treatment combination were studied more detail to assess the respective effect of pulse duration and power. Four repeated study were performed for each combination. From these studies we conclude: (1) YAG laser photocoagulation effectively stopped experimental gastric ulcer bleeding when applied at laparotomy. (2) Under the upper described condition of pulse, spot size and pulse duration most useful combinations for hemostasis were following: a) In 0. 5 sec pulse duration; 60 watt and 70 watt power. B) In 1. 0 sec pulse duration; 50-90 watt power. C) In l. 5 sec pulse duration; 40-50 watt power. (3) 3 out of 44 combination were perforated (7%) (4) Mean effective joules for hemostasis were 220. (5) There were no close relation between decrease of tissue injury, joules, power and treatment time.