IASL(iodo acetamide spin label) and MSL(maleimide spin label) disordered the orderly helix arrangement of myosin in the rest state of spin level. Especially the effect of IASL was great. The muscle was isometrically tetanized with three trains of 3ms pulses every 50ms between 5℃ with 25℃. Equatorial reflection change inferred that myosin head was moved to the vicinity of actin filament by spin level. The intensity change of 143a and 72a could offer information of the mass projection of population of myosin head along the filament axis. The slope of intensity profile of the mass projection of 143a and reflection of IASL is appeared and that of MSL is appeared sharply. The decrease of 215a reflection intensity the periodical character of 143a reflection by spin label. The raise of MSL actin reflection at 51a and 59a in the actin reflection change refers that the shifted myosin head binds a certain actin or changes an actin structure by spin label effect. Because iodo acetamide has a tendency to decease the actin reflection, actin dose not bind myosin head. From this result, we can conclude that IASL and MSL are spin labeled on SH of myosin head and disordered the helix arrangement of actin.