To examine relations between stellar activity and rotation we estimated parameters of stellar activity such as R′ H K , R′ M g I I , R′ C I I , R′ C I V and R′ X − r a y from the published data which measure the activity levels of stellar chromospheres, transition regions and coronae. In the present study we considered only the main sequence stars in an attempt to minimize the influence of other stellar parameters such as radius, age and stellar convection on stellar activity since they are also known to affect the magnetic field generation. In the present analysis we selected only those stars that satisfy the following conditions: (1) flux measurements are available together with Ca II fluxes and (2) rotation periods are determined by Ca II observations. We derived relations between the ¯Rossby number Ro and stellar activity R′ H K , R′ M g I I , R′ C I I , R′ C I V and R′ X − r a y and assessed the relations by plotting R′ H K , R′ M g I I and R′ X − r a y against rotation period P rot for comparison with observations. From the comparison it is found that as far as the rotation-activity relation is concerned, (1) normalized surface flux R′ H K is better than the surface flux F′ H K , in the sense that R′ H K differentiates the color dependence better and (2) R′ H K defined by Rutten (1984) describes the observations notably better than R′ H K of Noyes et al. (1984).

ABSTRACT
I. INTRODUCTION
II. STATEMENT OF THE PROBLEM
III. OBSERVATIONAL DATA
IV. RESULTS AND DISCUSSION
V. SUMMARY AND CONCLUSION
REFERENCES

• YUN HONG SIK(Department of Astronomy, Seoul National University)
• PARK YOUNG DEUK(Department of Astronomy, Seoul National University)