The present study was conducted to investigate the sensitivity of alfalfa (Hypera postica Gyllenha) to several insecticides in field conditions. Alfalfa is among the forage plants due to its good and delicious quality, having reserves including materials; various minerals, protein substances, and even a variety of different vitamins have a great important. Hypera postica is one of the most important alfalfa pests. If it does not decisively control it, sometimes more than 90% of the first-cutting crop of alfalfa is destroyed. This pest feeds on leaves and shoots at all stages of larvae and adult. However, major damage is done by larvae of the third and fourth instars. This research was conducted in field conditions in the city of Miandoab, West Azarbaijan, in 2016. The treatments consisted of, phosalone, cypermethrin, diazinon, deltamethrin, fenvalerate. Efficacy experiment was conducted in a completely randomized design with 6 treatments and 3 replications. Sampling from experimental units was carried out using quadrat. Moreover, the larvae population was determined in the experimental unit. The efficiency of treatments was calculated by Henderson-Tilton method. After analysis of variance, the average grouping was done by Tukey test. The results showed that the mortality rate, 15 days after spraying in cypermethrin, deltamethrin, phosalone, diazinon and fenvalerate were 96.92 ± 1.29, 92.41±2.59, 61.76 ± 2.23, 24.63 ± 2.48, and 24.58 ± 1.26 percent respectively. The percentage of mortality of cypermethrin and deltamethrin was significantly different with other studied insecticides. Diazinon and fenvalerate had the least effect. Due to results, cypermethrin and deltamethrin are recommended for controlling this important pest.