A sinus tachycardia is an accelerated sinus rate that is a physiological response e.g., to fever, anxiety, pain, exercises hyperthyroidism or cardiac disease. Heart rate is more than 100 bpm and rhythm is usually regular and P waves are present.31 cases (8 male and 23 female) has been diagnosed with sinus tachycardia according to the clinical feature and ECG assessment, ages between (19-34) years old, age mean ± SD (23.7097± 3.98492).Assessment of electrocardiography (ECG) to conform the sinus tachycardia, also assessment of Echocardiography has been done to exclusion others cardiac disease diseases. Laboratory investigations has been done, free serum vitamin D (25-dihydroxyvitamin D [25(OH) D]) has been evaluated by ELISA, this type provides the single best assessment of vitamin D status and thyroid functions (T3, T4, and TSH). Our results suggested that there is an association between the vitamin D deficiency and patients diagnosed with sinus tachycardia by assessment the free serum vitamin D (25-dihydroxyvitamin D [25(OH) D]). Also the results showed strong evidence among patients whose received treatment with vitamin D3 (300000 IU/month for 3 Consecutive months) and showed a good response according to statistical analysis (P = 0.005). In conclusion, the deficiency of vitamin D effect on regulation of the heart beats; also the poster of vitamin D 3 in certain dose for certain period can be treating the disturbance of the heart beats.