Being an available method of information fusion, image fusion has been used in many fields such as military applications, remote sensing, machine vision and medical images. Three image fusion algorithms are presented based on pixel-level including weighted mean of the original images, Toet algorithm, and the algorithm based on wavelet transform. Four kinds of quantitative evaluation criteria for the quality of image fusion algorithms are proposed such as standard deviation, average error, peak signal noise rate and the difference of entropy between the ideal image and the fusion image. Using the four algorithms to merge the fluorescence image and transmission image, the conclusions of the quantitative evaluation and the human vision are similar.