Climatic factors, seasonal distribution and the indications of sea mirage were d iscussed, based on the 37 sea mirages that took place over 27 years on the North Sea in Penglai. Results indicated that sea mirage may take place in Penglai all the year round, with a high frequency from early May to early August, for this time period had the most suitable climatic conditions. Temperature differences i n the air and sea surface reaching 8.5℃, windforce reaching 2 to 4 on the Bea ufort scale, and high humidity were essential to the occurrences of sea mirage i n Penglai. Fine days after rain or snow, great tide range, and a sharp change in air temperature were the indications of sea mirage in Penglai.