This research was aimed at investigating the effect of the application of varied dosages of guano compost on the growth and result of two varieties of onion. The research applied the Factorial Randomized Block Design (RAKF) which consisted of two factors. The first factor was the two varieties of onion (M), namely Bima Brebes and Sumenep. The second factor was the dosages of guano compost (G) which consisted of 4 levels of guano dosage, namely 0 g/polybag, 100 g/polybag, 150 g/polybag and 200 g/polybag. The F test followed by the BNT test was conducted to analyze the data. The results of the research showed that the treatment on the two varieties of onion (M) significantly affected the number of leaves and the length of the longest leaves on each observation (10 hst, 20 hst, 30 hst, 40 hst, and 50 hst), the number and the mass of fresh bulbs, the mass of dried bulbs, and the diameter of bulbs. The treatment of the varied dosages of guano compost (G) significantly affected the number of leaves at the age of 50 hst, the mass of fresh and dried bulbs, and the diameter of bulbs. Meanwhile, on the interaction of two varieties of onion and the varied guano compost (M x G), it significantly affected the number of leaves at the age of 40 hst and 50 hst, the mass of fresh and dried bulbs, and the diameter of bulbs. It could be concluded that the best treatment was on the variety of Sumenep and the most appropriate dosage of guano compost was G3 (200 g/polybag). Meanwhile, on the interaction of the treatment of the two varieties of onion and the varied dosages of guano compost, it was M2G2 (the variety of Sumenep and the Guano compost of 150 g/polybag).