Mushroom plays important role in economy and human diet. An attempt has been made in this study to examine the economic analysis of cost and return per 100 sq. ft. and input output ratio of Mushroom in Supaul district of Bihar. The study made use of a multistage sampling and randomly sampling technique to select 65 respondents among those selected villages. Data for the study were collected with the aid of a well- structured questionnaire. Data collected were analyzed using tabulation method along with required statistical tool. All respondent were categorized as small, medium and large growers on the basis of straw used in mushroom production. Resource use structure in mushroom was found to be varied among the size groups. Production cost of mushroom was varied according to size groups of growers. The per 100 sq. ft. cost of cultivation of mushroom was highest in small size farms and lowest on large size farms. The cost of cultivation was varied among the size groups of mushroom growers. The input output ratio is highest on large size farms and lowest on small size farms. The study showed that the per 100 sq. ft. cost of cultivation of mushroom varied from ` 2982.86 on small farm growers to 2387.54 on large growers with the average cost of ` 2676.25. On an average, total fixed and variable costs were ` 540.41 and `1474.31which constituted 20.19 per cent and 55.33 per cent of total cost, respectively. Average mushroom production per 100 sq. ft. area across small, medium and large growers was 56.23 kg, 57.77 kg and 58.65 kg, respectively.